In my experience (my 24 yo son is a heroin addict/5 years/12 rehab stays) when your gut tells you they are using, they are using. My rule is that the relationship cannot continue while he is using and so I have to cut off communication except for a call or text once or twice a month, just so I know he's still alive. When he is ready to try recovery again, I support that as best I can short of paying large sums of money for it. (He has good insurance... for now.)
This 'strategy' I have developed over the years is what I need to do for ME. I have found that detaching is the only way I can maintain my sanity, the only way I can sleep at night. Staying enmeshed definitely gives me PTSD.
My son has said that this detachment is also the best thing for him and is his greatest motivator to get clean. When he uses and his family detaches, his life falls apart damn quickly and is usually homeless within a month or so. This, too, is also a great motivator for him as you can imagine.
It took me a long time to get here -- to be able to detach in this way and accept that he could die at any time from his addiction. It's the hardest thing I've ever had to do, but I haven't found any other 'solution' that works as well for us. I guess it's not really a solution...it's just a way to stay sane through all this hardship and pain.
I can't tell you what to do or what would work best in your situation. I can tell you that being here at SR, therapy and family programs is what allowed me some measure of acceptance and serenity, and I am ever so grateful.