Lots of responses to this one! I don't know what's going on with your friend, but I learned recently that it can be really hard when your friends do shitty things to their partners. We were very close to a couple (in fact, we introduced them many years ago) and the guy recently started an affair with someone else and asked for a divorce. He did other dumb, hurtful things and has generally been clueless from start to finish about how shitty his actions are. I think our whole circle of friends feels like it also affects us, because he wants us to condone his behavior by acting like it's all no big deal. It's weird how the ripple effect is hurtful to us, the friends on the sidelines, too.
Anyway, I sympathize with your feelings. It is a terrible feeling to know that a longterm friend turns out to be able to do some pretty lousy things. The only thing you can do is to not let yourself be used. Be kind but don't get drawn in. My friend has tried to make me listen to his rationale and it just makes me feel soiled and upset, so I told him that I like him, but I also like his wife, and I just can't talk to him about it right now.