I've been in this situation before and believe me, if you try to tell her what a horrible person she is there is a 90% chance she will get mad at you. I also had a friend that was dating a complete loser. He was manipulative and condescending to her, but still she thought he was the greatest thing since sliced bread. When I tried telling her what I thought about him, she got angry with me and claimed that I didn't know him like she did, they were in love, blah, blah, blah. I learned then the meaning of if you can't say anything nice, don't say anything at all. She had to see his horrible ways for herself. Eventually she did and began the long process of breaking up and getting over him. I offered support whenever she needed it. In this situation, there isn't much you can do unless she actually asks what your opinion is of him. Whether or not she does, the most you can do is just be there for HER to lend support. If this guy decides to show her his TRUE self, she is going to need you.
I do have a positive story that's like this. I had a friend that dated a racist, redneck. I couldn't believe she would actually date him. However they stayed together, married and he really became a nicer person over the years; still a bit of a redneck but not as irritating and his prejudices are almost non-existant. I believe that she had a great influence on him. Of course, this isn't always the case but still a possiblity.