+1
The more he keeps his cool the better for him. If he can get through this whole thing without ever losing his cool then come legal time it will very much work in his favor.
It isn't unusual for the cheaters family to not approve of his/her actions. Her family may be honestly concerned, after 10 or so years they probably consider your bro family and are truly worried about him.
On the up side, he is in total control at this point. He did nothing wrong so legally, she broke the marriage contract not him, that "should" mean no alimony, a lot of leverage concerning custody, etc. As long as he doesnt get caught up in the emotions and do something unwise, he should be in a pretty good position if there is a divorce (and IMHO there should be).
He also needs to be prepared for her to come back when she realizes that the grass isn't greener and she discovers what she has lost. He needs to have a game plan for that event and STICK to it.
