Hi,
I didn’t want to read and run, and I know nothing about Ketamine, only that it seems to be more and more common.
Just know you are not to blame and you are not alone. It sounds like he needs to change his friendship group, as he can quite easily stop doing it (as proven by isolating) but then gives in to temptation. I dont know how addictive Ketamine is. Many people who use it, also use Cocaine, do you know if he is also using that?
Sending love x