The only idea I have is to tell you part of my story.
Once upon a time I abused drugs and alcohol. My boyfriend's parents spent the winters in Las Vegas, so we had their house to ourselves for several months. In our wildest DREAMS we couldn't imagine carrying on like that with his parents in the house, they wouldn't even allow anyone to smoke a cigarette inside. We resented it, but did whatever we wanted to do when they weren't there, and cleaned the house like hell the day before they came home.
We did exactly what we could get away with.
My boyfriend and I were hellbent on getting high, and we would fight (violently) and break up continually. The police could only intervene so far, and again, we did what we could get away with. I resented our families so much for "abandoning" us, for not "helping" us. I resented the police, the law codes, for not protecting me more, for locking him up and letting him out on bail. I resented my family for making me turn out like I was. I resented his family for making my BF like he was. I was angry at the world. Looking back, thank God I didn't get the "help" I designed, or I might have suffered even longer, or not be alive today. The help I was demanding wasn't help, it was a "rescue." I wanted it all to go away, poof! and I didn't want to face reality or my role in it, or take care of myself, etc etc. (My lack of self-care makes me a BlueRibbon Codependent, if there can be such a thing!)
Why did I tell part of my story to you? Well, because I think in some ways I've been like your daughter. I repeated the pattern in a few other relationships, until I did what I had to do to make it stop: MY recovery, all mine. I did the work, and God helped me (God didn't let me get out of doing what I had to do either!)
As a kid, I hated the scraming fights in my house. I feel for your 10 year old too. Its so hard for kids to be subjected to stuff like that. When I was an immature drug/alcohol abuser, I had no sense of my affect on others, and no sense of responsibility toward children in my family or anyone elses. I know how empty and spiritually bankrupt I was, and I feel sorry for any child who has to be around someone like that. So don't leave the 10 year old's safety in their hands! Violence escalates, in my experience. I'm not trying to be judgemental toward you, I'm just concerned and worried.
So what can you do about your daughter and her BF? You're right: you didn't cause it, can't cure it, can't control it, but you can set healthy boundaries. Reread your post, look at it objectively, like it was a TV script. What's your role, what would you say your "character" is all about?
When I think back to my "old days" I can only imagine what it would have been like to bring my BF home, sleep with him, get high -- basically do as I pleased, regardless of the consequences or example I was setting. You're giving your daughter and her BF a lot of space to operate. Boy, did me and my BF get pissed off at anyone who didn't "respect" us and our (sick) relationship. Lots of tears, lots of angry outbursts, hurt feelings, around and around we'd go. In retrospect -- I'm glad for the honesty some people had to simply NOT PARTICIPATE in our sick sideshow!! We had no respect for ourselves or others, and bumping up against those boundaries helped me get out of that life which was leading me nowhere. Your 10 year old deserves and has a right to role models that will benefit him/her in the long run. What is your 10 year old going to absorb from this interlude?
I thank God that I am no longer addicted to drugs. I don't know why my HP helped me with that, because I was sure I couldn't ever kick it. I was extremely selfish. I was addicted to drugs, and to "love" from my violent boyfriend. I wanted what I wanted, period. Never mind any of my good qualities. The high and the BF came FIRST, and I fiercely resented anything or anyone who called me on it. I was angry and self-destructive, and very determined to do things my way. Addiction obscures and eventually destroys the good in a person. Since its a family disease, it can destroy others too. And no one person can stop it from happening to another.
Are you trying to be that one person who can?
Take what you want, and leave the rest. Love, prayers, and hugs, candlelight