Hindsight is twenty twenty. When she volunteered to open her legs to have unprotected sex, all she wanted was a orgasm. If that was all she did, maybe she could keep her mouth shut. But she a kid now and it ain’t her husbands. The Child has to be taken care of and it’s her responsibility and the fathers. If the husband wants to help, ok. If not, and she keeps it a secret..then the child suffers now or later.
She could have had an orgasm with the husband but she cheated on him instead. This decision was not about sex, she deliberately chose to cheat on her husband.
I'm presuming here that you're a man.
Knowing what I know about women from associating with them so much, I'd say it's safe to say that women don't cheat just for the sex (unless they're getting little or none in their marriage). I just read this article recently and I'd say he's got a pretty good handle on it.
I Went Undercover on Ashley Madison to Find Out Why Women Cheat.
http://www.dailydot.com/opinion/ashley-madison-why-women-cheat/
That said, there's no justification for breaking your marriage vows. No woman should ever think they're above being tempted in that area and that it wouldn't or couldn't happen to them.
I'm going to presume here that she was not at home as she already had a child. Could be that she put the kid to bed and then did the deed, but I'm guessing not. If I had to guess, I'd say she put herself in a compromising situation.
She made her decision. She should not have been alone with her husband's best friend when he wasn't around.
This is the price you pay for that kind of behavior. Marriages can heal from that kind of betrayal, but I honestly don't know what my reaction would be if I found out mr. mm had ever cheated on me.