He's a total asshole! I think you should really try to get your friend to believe this, but I don't think you should interfere too much.

Oh, also, this dudes also been convicted of rape twice, and is still under investigation in both cases.

What a surprise.

Shouldn't this be enough to warn her?

The lies and ruining rep stuff may be hard for her to believe, but the rape/ convicted thing has to be a deal breaker.
If it isn't then the girl is practically asking for trouble so leave her and the dude alone.