It's glaringly obvious your good friend is in love with you and is envious of your fiance. Does your "good friend" talk negatively about your fiance when he's not around? He has probably been passive aggressive (and sometimes just aggressive) toward your fiance since day one. You won't be able to keep both of them in your life in a meaningful way.

Also, I'd say that your fiance is justified in standing up against "nice guy", steal-yo-girl BS.