My recommendation as to what kind of friend: be a man with strong convictions about marriage, adultery, faithfulness. Don't let your desire to mend fences with c lead you to compromise on your convictions. Definitely do not agree with her wrong approach to marriage (ie., ok adultery cause her h is mean, whatever) as an attempt to validate.
If she says well you weren't like that before, say well I've changed since all this began, I realize now what is important to me and I'm standing on my beliefs.
Don't be afraid to have convictions that may not be what she wants to hear. IMHO.
