T.D.
That is a tough one, knowing how I was when I was that age, nothing my parents said made one bit of difference to me. As a parent myself all I know now, is that I really wish I would have listened to them....LOL
I think my best advice for you is just to support him in his decisions. Be there for him, and at the same time, explain to him that you don't feel she is the best person for him. I know how hard it is for parents to sit back and watch as our children make decisions we know is not good for them, but on the same hand, I know as well, that having paid my own consequences of not heeding the amazing advice of my parents, that he will realize that you are only out to see that his best interest are met. He will make the right decisions eventually, and it will be because you took the time to support him, and listen to him, and offer your opinions to him as well.
Just let him know how you feel, and what your concerns are and let him take it from there.
I would also maybe take the time to talk with her, a little. Nothing to obvious, maybe a few questions here and there about things you need to know, or things that bother you. Maybe this is the only way she knows how to be, and sadly, maybe she hasn't had the posotive influence in her life, that you have given your son, maybe she is acting that way out of envy for what he has and she doesn't. It's so hard to tell what sort of support someone has or hasn't had in their lives.
Just my opinion......Hope it helps!
T.