For any number of reasons a woman might become entangled with a married man. In a perfect world the only person who becomes 'entangled' with a married man is his wife but our world is not perfect and we should reserve judgment.
How or why would a woman become involved with a married man? One reason could be that he lied about his status. Sometimes women deliberately seek out married men or a married man is in the process of being divorced and decides not to wait until it's finalised before moving on with his life.
Naturally you don't want to be deceived or duped into thinking that he is single when in fact he's married. If you fall for a married man because you were deceived into thinking he was single then you should leave upon discovering the truth. Leaving, I know, is easier said than done when you have fallen in love. I do understand that at times when a man is going through a divorce his wife can be exceedingly vindictive and drag on the proceedings particularly if she discovers that he's already seeing someone or she didn't want the divorce.
At times a man is married only by virtue of the marriage license (paper marriage); he and his wife lead separate lives, sleep in separate beds, may not be living together, are only together because of the kids, a financial arrangement or the opportune time for a divorce has not yet arrived so they remain together until the ideal moment when the divorce can proceed. Trapped in a marriage is not only applicable to women it is a legitimate nightmare for some men. Some women can deal with his situation while others cannot, it's an individual choice.
If you have fallen in love with a married man, regardless of how it came to be, you should present an ultimatum particularly if he has also professed his love for you. At some point, sooner rather than later, he should have the divorce finalised. While to him it's only a marriage on paper, to you and your friends he is a married man and the stigma brought forth is not dependent upon the intricacies of his situation.
Your ultimatum depends on you, how long can you or should you continue seeing him until he is finally divorced. In some cases, he sleeps at your house, you see him constantly and you get the sense that even though he is married he makes or has time (plenty) to spend with you. Such a situation could be evidence that it's really a paper marriage or he's a good liar, his wife believes he's is somewhere legitimate.
I do not believe in confronting someone's partner so to me it's never a good/smart idea to confront his wife. Sure, if you were duped you might want to make his life miserable by telling her he has been or is unfaithful but that's totally up to you. The focus, if you were duped should be on ending the relationship so that you can move on.