Naranon and alanon are meetings for people who love people who are addicts or alcoholics. I have never been to one of those meetings but I have heard nothing but good things about them.
She could get a meeting list by phone for Narcotics Anonymous meetings for her son. But, if he doesn't want to quit he probably won't be open-minded about what he hears there.
Telling her son that she knows about his drug use was for the best because denial of the problem won't make it go away. But, I know from experience that ultimatums don't get people to stop using, unfortunately.
She will have to go with her heart, but in my opinion, being a "co-signer" to someones addiction won't help them stop. In other words she shouldn't put up with drug use in her home.
My mom was aware of my drug problem fo quite a while and by trying to keep me from the consequences of what I was doing, it only prolonged the inevitablities of what addiction brings. For example: She gave me money so I wouldn't steal to get drugs. I still did. She let me stay at home so I wouldn't be on the streets. I still was. She coddled me so I didn't have to deal with the consequences, but it was the consequences that led me to stop using. The price to continue using got too high.
She might want to look into local detox places also. Even if someone doesn't really need to detox for withdrawls, they give someone the chance to stop the using cycle long enough to have some clean time and be able to make better decisions for themselves.
I'll keep you all in my prayers.