How do you deal with someone who has a meth addiction?

We just found out that my brother in law has been a meth addict for eight years now. He is single handedly destroying his family. He lives at home with his mom and has never had a job. She’s a wonderful person but I don’t think she knows how to deal with this. He told her that he doesn’t want to go to rehab, so she won’t send him. She seems to think that he’ll stop on his own, or maybe she’s just resigned to the fact that he’ll never quit. What can we do?