Should I tell my friend how I really feel or just let her marry him?

My best friend is engaged to a Mexican citizen currently living in Mexico. They met when he was working up here illegally. They dated for 6-9 until he moved back to Mexico. She would occasionally go down to Mexico to visit him. They got engaged in December 08 and a January 10 wedding in Mexico is planned. She was going to live down there for 2 years after they are married so she can legally adopt his two boys (3 yrs and 4 yrs old) and legally bring them back up here and he will somehow come back up here and they will all live up together. I had no problem with this or him when I found this out. But then a couple of weeks ago she calls me from Mexico. She had been down there for a month and was planning on staying two more weeks. She told me she was absolutely miserable. He has only worked 5 days out of time she has been down there. He spent all the money she brought down. He hasn't done any part of the wedding planning he was supposed to do (booking the church and reception place). He makes her clean his house all day and yells at her if its not clean. He leaves for hours at a time (4+) and doesn't tell her where he is going (or lies about it) and won't ever bring her with. He calls her “stupid” if she doesn't understand something (she barely knows Spanish). He smacks her so hard on the butt that it leaves handprints and he keeps doing it even though she has told him to stop. He has started hitting her in the arm and calling her “stupid”. She told me that he is not the man she fell in love with and isn't in love with him anymore. And it gets even worse… I found out that in the two years since he moved back to Mexico she has sent him over $20,000 and he blows it on random stuff like sunglasses and DVDs. He once hit a woman with a belt and when my friend said, “you don't ever hit a girl” he said “well she deserved it”. And the reason he can't come back into the US legally is because he got caught selling cocaine 3 times and was deported. When I got done talking to her she said she didn't think she wanted to marry him anymore. She just got back a couple days ago and is still planning on marrying him. I haven't had a chance to talk with her about this since she got back because it wasn't the right time or place for the conversation. I am supposed to be going down to Mexico for their wedding and be a bridesmaid but I don't know if I can stand up there and pretend to support this marriage. I don't know what to do. I can't let her marry this poor excuse of a man.