My best friend has a boyfriend that I CANNOT stand.

I could go on and on about why I dislike him as a person (poor hygene, lack of intelligence, failing school, into drinking and drugs, has been suspended, lacks basic social norms, etc), but even worse is how he treats my friend. He monopolizes her time, tells her not to go out with her friends, only to hang out with him etc. He pressured her into having sex for the first time, and in all honesty I think he's just using her for sex. She knows how I feel and appreciates my opinion but told me just to butt out. I don't know what to do. Is there anything I can do?

My friend lives with only her mom (who is an absolute nutcase, but that's another story), and her mom is completely unstable, and I think my friend is just looking for something stable... I don't know what to do. Help.

By the description, he is not stable. He's the opposite.

Talk to your friend, and have a lengthy discussion. A good one.

Talk to her, and tell her why they shouldn't go out. Give reasons, and then explain your opinion and how you feel on the subject.

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I've been in this situation as well. Its not easy.
But since your friend has told you to butt out, unfortunately there is really nothing you can do. Unless her boyfriend is physically abusing her or getting her into drinking or drugs (in which case you need to talk to her and if she fails to respond talk to an adult) you really do need to let her make her own decisions. She'll resent you otherwise (even after the breakup) for not letting her make her own mistakes and learning from them.

I'm not saying you cant say anything, just limit how much you talk about him negatively to her.
Having a serious talk about him to her is a good idea, but after that just be there for her, and if he does something out of line point it out to her, but keep it at that.

Sorry if that didnt make lots of sense, I was just typing thoughts as they came to me.

wow thats pretty harsh..
it sounds like peer pressure
just warn ur friend how you are feeling exatly and express before she rly gets hurt tell her that she is thinkign about now and not next

just talk to your freind and ask what she sees in that guy