Likelihood of Meth Addict to Come Clean?

I know someone who was sent to prison for making/selling Meth. He has been out of prison for about 3 years now, and has been clean for about 6 months. He has never gone to rehab.

How likely is it that he can stay off drugs long term? I thought this was a really hard drug to kick….? Can someone just quit cold turkey?
Don’t worry, I have nothing to do with this person. I just need to know for legal reasons.
Ok..once again…this is NOT my friend. I need to know for LEGAL reasons.