Am I wrong to give an ultimatum to my boyfriend about his pothead friend?

I have been with my boyfriend for 5 years, and living together the last 2 years. He had a serious drug addiction to meth and pot, and went to residential treatment for a little over 6 months- Feb. ’07 to Sep. ’07 and came home. The problem is we have a guy friend “C” who is a really good guy, but a SERIOUS pothead and smokes weed all the time. While my bf was in rehab, my bf even said that he doesn’t think he can be friends or hang out with “C” anymore. When my bf got back from rehab, a month later I allowed him to hang out with “C” at C’s house almost every weekend. I don’t know if my bf is lying or not, but he said that while “C” would smoke weed, he would go outside and have a cigarette. I trusted my bf in hoping he won’t get addictive cravings and start weed again, and I trusted our friend “C” to watch out for him. Last Sunday, I caught my bf in C’s truck, and it was all smoky inside. My bf claims he didn’t smoke weed and was just talking. I told him I would buy a drug test from
the drug store, but he said he didn’t think he could pass it because of the “second-hand smoke” from inside the car. I told him that from now on, he and “C” would have to hang out at our place- I also gave him the lecture of how rehab is the only thing he’s accomplished in a long time and throwing it away. I told him he can make his own decisions, but I will not ENABLE this drug addiction and he will have to move out! I wrote “C” an email explaining the situation about hanging at our house and the reasons for it, and “C” wrote me back saying that I’m being a parent and controlling! It was a very offensive email! I don’t know what to do, I don’t want to be responsible for this but I can’t enable my bf to go back to being an addict. Am I wrong?
I also want to add my bf has not had stability, and not been able to hold down jobs because of his past addictions. He is also on probation and had to do the court drug-treatment program, so probation can still drug test him probabl for the next couple of years. Our friend “C” knows all of this but is still not understanding! I don’t care if me and my bf stay together- I just want to give him a chance at finally accomplishing something, having a career and being self-sufficient.
Thanks for all the answers- it really helped me think about everything. Had a serious talk with the bf and he actually agreed with me. He says he will talk to our friend “C” about new arrangements for hanging out- and if “C” can’t understand- he’s not a real friend at all!

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