If she has no remorse, i don't think you'd be unreasonable to tell her your true feelings (or at least you don't want to hear it), but she may be unable to feel sorry for the wife since he probably paints the picture that she's the worst human in the world. Your friend sounds very insecure and probably slags the wife off because it makes her feel better.
I was in a similar position as your friend; I was messing around with a guy that had a fiancé and young child, although, I was a much worse person since I was cheating too, but needed some excitement from my shit relationship and was very insecure (I know, not an excuse). He spoke about his fiancé in such an awful way and said the only reason he was with her was their child (obviously lies). It fizzled out, but karma had got me a few months later when my partner at the time left me for someone else.
I found an amazing guy, and was completely honest about my past from the offset since the way my previous relationship had ended put things into perspective for me.
I fell pregnant and a friend told me she was doing the same thing as what your friend is doing (relationship with married man, wife was pregnant, etc), I felt sick, couldn't understand why he would do it, why my friend would do it knowing how it feels to be pregnant and betrayed. Her reaction was 'All men cheat anyway, why be cheated on when you can just be the other woman and never get cheated on'. Id lost all respect for her.
I already knew I was wrong in the past, but thinking of that poor pregnant girl and picturing myself in that situation made me feel terrible.
I also dumped another friend that egged on my situation when I was in it. She felt better I suppose since she was doing the same thing to a married man, too.