I'm wondering why his wife decided to cheat on him in the first place? There is always a reason to drive another mate to do this....
I dont agree with that (at least not in my little bubble of a world

)
There is no excuse to ever cheat.
If things are that bad then you get divorced first.
When people cheat they will make up all kinds of excuses as to why, some might be understandable, and other excuses are just down right B.S.
The bottom line is cheating is the ultimate in selfishness.
And naturally only one side of it is being heard.
his wife could have left him because he was a drunk and abusive, might not have ever cheated on him and he could just be saying that either because he is imagining it, or he knows he maybe he screwed up and is trying to make her look like the bad person.
I guess the bottom line is there are always three sides to every break up, his side, her side and the truth.
Stay out of other peoples relationships if it means taking a side and hurting someone. If you can be helpful as far as lending a shoulder to cry on, moral support etc. go for it. but make sure it is known you are not taking sides.
And when it comes to legal matter and going to court for someone. they need to be a really really good friend and they need to have their ducks in a row.
I wont even be a character witness for a family member if I think they screwed up and deserve what they are getting.
So yeah, my advice. stay clear