Dagny was fully aware that Hank was a married man, yet she still chose to sleep with him. Does that mean she is also responsible for "helping" Hank do evil by going back on his "wedding contract" with Lillian?
The moral issue of justice always depends on the full context. (More broadly, rationality in general always depends on context. All knowledge is contextual. At the most basic level, awareness of context is, in fact, the essence of free will.) The context of the story in Atlas Shrugged makes it clear who is the victim and who is the oppressor. The biggest error involving Hank and Lillian in the story was Hank's, for not divorcing her far sooner. But if he hadn't been characterized as committing that error, the story wouldn't have been as dramatic and insightful as it is.
Update: Sanction of the victim
A comment questions whether Hank is a victim, since he sanctioned it, by his own choice. Yes, exactly. Hank gave his own moral sanction to his attackers, and they seized on it. He represents one of the major concretizations of "the sanction of the victim" in Altas Shrugged, not only in his relationship to Lillian, but in many other aspects of his life, as well, including his brother, his mother, and his political oppressors. In abstract terms, his error was in granting his moral sanction to an anti-life moral code. His error allowed him to become a major victim of that code, until he gradually learned to shrug it off.
Note, also, that he did not pretend to be in love with Lillian. He stayed in the marriage entirely out of a sense of duty, not love, thereby concretizing the ugliness of "duty" as a moral ideal.
(Another comment invites elaboration on my remarks about free will, and this certainly merits further discussion. But it would be better to discuss in a separate question thread rather than here. I will probably post such a question myself if the commenter doesn't do it first, but the commenter's own wording probably would be more indicative of the commenter's specific context.)
Update: Victim versus Victimizer
A comment asks for a critique of the following formulation:
1.Subjectively, the marriage contract exists (in the minds of Lillian and Hank).
My assessment is that this formulation omits far too much of the total context of the story in Ayn Rand's masterful, epic novel, Atlas Shrugged. If one wants to judge whether Lillian Rearden is a victim of injustice or a key perpetrator of injustice (principally against Hank), there is abundant, concrete evidence provided by Ayn Rand throughout the story.
I also see the commenter's formulation as an attempt to cast moral issues into the form of a narrowly deductive (syllogistic) legal argument, although the commenter may disagree about this. Morality is far broader than an equivalent of rules of law. I can discuss the nature of morality further if it would be helpful to anyone conscientiously seeking to understand Objectivist philosophy as it is, in its full context.
The use of the term "contract" in relation to marriage in the commenter's formulation also omits (in my view) the fact that marriage contracts are open to divorce (legally speaking, if not necessarily in conventional morality), and often ought to end in divorce, and often sooner rather than later. I see Hank's situation as precisely one of those cases. Hank Rearden certainly is no embodiment of consistent moral perfection; he is depicted by Ayn Rand as a hero of mixed moral premises, heroic in his professional life, but a helpless slave in his personal life. He made his own enslavement possible, but he didn't know it until he gradually learned better. How can any rational moral observer possibly hold Hank to a relationship that he, in perfectly good faith, misjudged so badly from the start? It was never a mutually romantic, rationally loving relationship, despite Hank's best intentions. I say let us not confuse the victim with the victimizer. It's not some kind of "close call" based on the moral status of breaking a marriage contract.
Update: Objectivist view of Marriage
In the original question and most of the follow-up comments, considerable fuss has been raised over the concept of marriage as a type of "contract." One of the terms of such a "contract" is conventionally considered to be unbreached fidelity for as long as the "contract" remains in effect. Since Hank violated that "contractual requirement," he is alleged to have "breached the contract" (unilaterally), which is regarded as an injustice toward Lillian. Dagny, in turn, is alleged to be an "accomplice" in that injustice. Objectivism also classifies unilateral breach of contract generally as an indirect form of initiating physical force against others, which Objectivism regards as morally evil. In remarkably succinct, essentialized terms, the original question asks if Dagny "is also [in addition to Hank] responsible for 'helping' Hank do evil by going back on his 'wedding contract' with Lillian."
I maintain that this is utterly fantastic to anyone who has truly read Atlas Shrugged and understood it. I see it as massive context-dropping (and rationalism). Ayn Rand portrays Hank's marriage to Lillian as ceasing to be a morally valid relationship very early in the story, and Lillian is depicted as a key looter and evildoer toward Hank, like all the other looters whom Lillian strives to socialize and fraternize with. She is about as far from an "innocent victim" as anyone could ever get. Any claim of a still-binding "contract" between them is depicted in the story as irrelevant, since the marriage had long since ceased to be what Objectivism holds that a marriage ought to be, virtually from its very beginning. Their marital status is treated as significant only as emphasisizing the conventional, altruistic view of morality (and marriage) by which Lillian has succeeded in enslaving Hank -- until he learns to shrug. Their marriage is portrayed as one that ought never to have existed at all and ought to end as expeditiously as possible, with any marital commitment between them completely nullified and morallly unenforceable (in the moral perspective from which Ayn Rand develops the story).
In Part I, Chapter VI ends with Hank explicitly wondering to himself why Lillian married him. "What did she want from him? -- he thought. What was she after? In the universe as he knew it, there was no answer."
In Part II, Chapter II, in the section that begins, "Money is the root of all evil," at the wedding party for Cherryl and Jim, Hank sees Dagny and silently wonders:
... who had the right to demand that he [Hank] waste a single irreplaceable hour of his life, when his only desire was to seize the slender figure in gray [Dagny] and hold her through the length of whatever time there was left for him to exist.
Surely Ayn Rand is intimating here that he should have pursued that thought.
In that same sequence of scenes, there is also a conversation between Lillian and Jim in which Lillian describes her "horsewoman" perspective and her aims in regard to the "horse" that she married. She also informs Jim that she can "deliver" Hank to Jim anytime she chooses. She has that kind of power over Hank.
For a still more devastating critique of Lillian, refer to Andrew Bernstein's CliffsNotes booklet on Atlas Shrugged, p. 24. He describes Lillian as pure evil, seeking to destroy the good for being good.
For more on Ayn Rand's view of marriage, there is a brief but very concise excerpt in The Ayn Rand Lexicon under the topic of "Marriage." Refer also to the related Lexicon topics of "Love" and "Sex" for a fuller understanding of Ayn Rand's philosophy concerning those topics and their relationship to marriage.
Update: Hank's Immorality
A further comment asks, in effect: what was the nature of Hank's immorality, if any?
My answer is that the principal moral issue regarding Hank is the "sanction of the victim." There is some dialog in the scene from Part I Chapter VI that addresses this issue directly. Francisco is having a conversation with Hank to get to know him better:
[Hank says to Francisco:] "... the man who works, works for himself, even if he does carry the whole wretched bunch of you [including Francisco, in Hank's view] along. Now I'll guess what you're thinking: go ahead, say that it's evil, that I'm selfish, conceited, heartless, cruel. I am. I don't want any part of that tripe about working for others. I'm not."
Was Hank acting in full accord with a rational morality in having a romantic relationship with Dagny while still remaining married to Lillian? No, not quite, although Lillian, as depicted by Ayn Rand, was so thoroughly evil that she really didn't deserve better and didn't want or expect better for any romantic reason. She never cared for romantic love at all. Hank, however, was deeply torn between altruism in morality and the needs of man's life. He was Ayn Rand's concretization of the "problem of temptation" that infests altruism. In Hank, she shows what the mind-body split means in very concrete, dramatic terms.