I haven't and won't bad-mouth his wife. I don't know her, and I don't have a problem with her except that he's been unhappy with and bored by her. But that's his thing, and I try to not mirror other people's feelings-- as in, I don't choose to dislike someone just because someone I care about feels that way. Of course if I did dislike her for my own reasons, that would be different.
Oh totally. It's just very easy to let sympathy slip into ... something a litlte more. I did it a couple of times with my last b/f and it made it harder to relate easily to his wife when they weren't having problems. And during a divorce, he'll swing back and forth between missing her and hating her and everything in between most likely. Just remember that what he says right now might not reflect what he feels in 10 mins.
I know I could go from being pissed off beyond belief at my husband to being weepy and sad in the space of seconds. I'm sure my b/f thought I was a basket case. *wry smile*