This won't end well.
Unless her husband is okay with you two getting together, you'd better seek another woman. If she's unhappy with her marriage, she needs to end it. Yes, there's a difference between "ending it" and "keeping it alive whilst cheating".
If she keeps her current marriage alive while you two meet up (for whatever purposes), none of you will have a full relationship.
You'll be "with" a woman who is partially devoted to her husband.
She'll be with a secret "friend" (you) while she tries to stay married to someone else.
Her husband will be in a half-marriage where her heart's not in it, then he'll find out about you two and probably beat you senseless, if not a more extreme version of that.
As much as I hate saying this, don't do this. It really isn't worth the trouble.
I am homosexual. A lesbian, actually. If you have a problem with that, then it sucks for you. It's not your problem. It's not even a problem. I quite like it this way.