Ok I will try not to put too many identifying details on this as I don't want to be outed.
DC has a close friend and we are friendly with the family for just under a year. Or at least we were until this week as it turns out the H has been having an affair and has walked out.
She is being amazingly brave and is coping as best she can and trying to appear amicable for the sake of their DC. I'm doing what I can to support her and she has a good network of friends.
Fair enough no reason why I have to be nice to him I can't bring myself to be rude but as its all so soon I have managed to avoid speaking to him so far at school gates etc. Not DH he feels sorry for him 
and is extra chatty and may end up going out for a drink with the guy next week ( they have never been out for drinks before).
Now DH is totally faithful and claims to be disgusted by his behaviour, but says that he is still a human. I agree that he is still a human but one who is prepared to walk out on his child and wife for a younger model with no baggage and as such I will be cordial but nothing more.
I feel irked by this, I can't and won't dictate what DH does but it feels disloyal to me as I have sided with what I believe is the wronged party. Do I just need to get over myself?