That is a horrible situation, kind of similar to mine, my mother is engaged to a man-child at the moment and I can't stand him, he doesn't like any of our family and he's really creepy, his family are all mental and they just give my mother stress all the time but she is blind because she loves him, even though a few times he broke up with her because his sister told him to.
My mother and him are in their 50's but they have no common sense. I've told my mother all of this and she didn't listen, she doesn't care that we are trying to help her see him for who he truly is. Anyway I think it's a bit different in our situation because none of us really depend on her and haven't for a few years.
I really think you should tell him though, if he's marrying someone he should know all aspects of her, it's only fair, even if he doesn't take it on board straight away he'll probably realise you were right when she does something unforgivable. I think it's way more complicated taking the wedding into consideration though. :S