I can sympathize with you on your situation. I have been divorced from my daughter's father since Dec. 2000 and they have had no contact with him since November 2004.
My oldest daughter (who is now 23) moved out of the house right after her high school graduation in 2003. She moved in with a guy she met while they were both working a temporary job for our local newspaper. At the time, he was living with three other guys that weren't his blood family but he considered them his family. This was my worst nightmare but knew she was 18 and could legally do it without my permission. I knew nothing of his background, how he grew up, or even where he came from. He had tatoos all over, smoked cigarettes, and marjiuana. Needless to say, he was not my choice for my daughter. They have now been together for 5 years (married 4) and have given me a granddaughter and a grandson. He has not worked much since she moved in with him. I used to think he was 'allergic' to work and she is still the only one working. But now he has a medical reason for not working. At age 30, he had a blood clot in his lung. The hospital finally diagnosed him with COPD (chronic obstructive pulmonary disease) and susceptible to blood clots which are inherited. He will be on blood thinners the rest of his life. His parents have nothing to do with him. I have learned that when he was 14, his military dad put him in juvenile detention and moved out of state without telling him. His mom did nothing to stop this. They left him behind. To this day, he has nothing to do with his dad and very seldom talks to his mom. My daughter has heard him tell his own mom that I treat him more like a son than she ever did. To make a long story short, I let my daughter make what I thought was a mistake but it turned out for the best so far. He is a good father. My 3 1/2 year old granddaughter knows her colors (without hesitation), most of the alphabet, and some numbers. He stays home with the kids while she works and teaches the kids to talk and things they will need to know before going to school. I know things could have turned out differently but glad they didn't. She still calls me to complain about him or work, and I just listen. I can now give suggestions, but it's still up to her what she does about a situation she doesn't like.
I know it's hard but we have to let our kids make their own mistakes so they will grow up. It seems harsh and cruel, but at 18 they think they know everything and have the legal freedom to do as they want. Turning 18 and being able to do as they please is exciting to them. So they won't be thinking ahead of the consequences that could occur. The only thing we can do is to be there for them, no matter what happens.
My youngest daughter just turned 18 in May, so I'm going through this all over again. At least her boyfriend works, but I also know nothing of his past except that his dad has beat his mom up on occasion. That's not a situation I want my daughter in either. All I can do is to make sure that I'm here for her and to listen when she wants to talk.