As someone else mentioned, it probably would have been worth giving her an opportunity to give her perspective before deciding if and how to tell the husband. Most likely the final result is just telling him, but there are more marginal possibilities that it is worth accounting for and taking an extra couple of days wouldn’t have harmed you or him (yours was already a dead relationship by virtue of your understandable disapproval of her cheating, so it’s not like you’d be enabling her to cheat for longer unless you think she’s managing to juggle even more guys along with young children and a marriage).
Talking about this in terms of social contract theory is really sidestepping the morality of the issue. Would you say that her lying entitles OP to punch her in the face? Surely not, two wrongs don’t make a right, punitive justice is bad, etc. What OP should do is investigate the issue with her not because she “gets to” tell her side or has a right to, but because he doesn’t know what the consequences of telling the husband would be. For all he knows, the husband is abusive and would beat her for this transgression, transgression though it is. The most likely outcome is, of course, that the husband is not abusive, but the most likely outcome of a round of Russian roulette is that you go unharmed. In either case, there is a real risk that is severe enough that it’s worth checking, even if it’s substantially less likely.