Well, the whole thing is weird, isn't it? I mean, trying to help a person in need. Isn't that suspicious?
Note: sarcasm filter is now off.
And I'm sorry as I admit I was being flip there. He doesn't mention this woman 24/7 to you so, naturally, there would be some time in between mentionings. He only seems to get involved in her life when she's in dire need. So he might feel the need to be something of a white knight. Which is usually kinda misplaced (and in the case of the nasty husband, downright dangerous).
Both of those situations called for professionals, yet your boyfriend decided to handle them both on his own. Being a friend is one thing, but the nasty husband is the reason why cops exist, and the woman being suicidal is why psychiatrists exist.
His best bet is to urge her to seek counseling. It would be better for her, anyway, to work with an actual professional and not unfairly lean on her friends and put them into a situation way above their pay grades. And it would get him to start breaking the cycle of riding in on his white charger. He might want to get a little counseling himself, in order to see why he's doing this.
But no, I don't think he's cheating or anything of the sort. And I don't think she's trying anything, either.