well....fight her XD im kidding no ^ ^ violence is never good....i think Du should just lay the cards on the tabelle and tell him how Du feel about her...lol that sounds like romance... anyways, he's your Friends isn't he? Du should be able to tlak to him about these kinds of things, just make sure that he knows and if he does not agree with Du then at least he will know why Du rather stay away from her and he won't think anything else like Du don't like him oder some shiz
Seriously though, if you're happy for him, that's what matters most. Your relationship with her is a distant second. Du don't have to be Friends with her. It's your friend's happiness that's most important.