My sister has done this exact same thing, multiple times. And she's stubborn as all get out and would just get angry if we expressed concern or dislike for her loserfriend. What she did do that was smart was refused to let him pay the mortgage - he paid the bills which were in her name. It at least prevented a claim to co-ownership of the house. My mother tried to stop supporting her, but would cave in order to prevent bankruptcy or loss of the house. 

What eventually happened (all three times... ugh) is that she did finally admit that the fact that he loafed on her couch all day while she worked sucked out the love and life of their relationship. She eventually dumped them and then admitted that the guy was a loser who treated her badly. 

I think all you can really do is wait until she's ready to talk about him. If she knows you don't like him, she'll eventually turn to you to complain about him and then you can be gently supportive of the idea that she break up with him.