I definitely agree with everyone that said to be a good friend to her, listen to her, let her cry on your shoulder and don't say too many bad things about him but lots of nice things about her. BECAUSE, if they stay together and you have completely trashed him, she will not be comfortable around you anymore knowing that you think her hubby is scum and she will feel like you think she's stupid for staying with him and she won't want to be around you. Trust me on this one
I do think he wants out and doesn't know how so he cheated and was sloppy about it on purpose. No one leaves their email open-even if they aren't hiding anything. WHo the hell leaves it open when there's incriminating evidence on there??
If you have the room, offer her a place to stay until she and loser boy make a final decision. She needs to play hard to get. She should not be the one begging him to go to couple's counceling. It should be the other way around. Honestly, if he is not groveling and begging and crying and calling her 1000 times a day to apologize and all that-he doesn't want it anymore. I hate to say that but I really feel if he was truely sorry, he'd do ANYTHING to get her back.
A condition of his probation should also be that he invite The Homewrecker over and formally break up with her in front of the wife. That way, wife knows that they are officially done, Homewrecker knows it's over and hubby knows it.
And I am NOT saying this is her fault or anything. But people don't usually cheat because they are mean hurtful people (Yeah some do) but because there was already something wrong in the marriage. There may have been other stressors besides the whole brother thing that we just don't know about.
Either way-you are smart girl and a good friend to her and that is the best thing she can have right now. God bless you!