I've been in this situation three distinct times, and I've told my "friends" that they absolutely shouldn't marry this person, and I was of course ignored. Example: In one of the situations the groom 3 months before the wedding "fake" karate chopped his fiancee in the neck (to the point of obvious pain for her) during a birthday party with about 15 of his friends present. He thought it was funny, the rest of us looked on in horror. We'd had our doubts before, that pretty much sealed the deal that the guy had major issues.

So now I simply refuse to bother interfering with other people's relationships, or even really give advice, because frankly they aren't going to listen anyway, they are looking for validation, not advice or knocking them off their pedestal.