You already know this, but I’ll reiterate it in case it helps you get over whatever guilt you might be feeling about it: you can’t. If you have already offered them a non-judgemental space to vent and have expressed that you’re there for them, then you have already done more than any friend should be expected to.

You say “it doesn’t rest in [your] hands alone”; it doesn’t rest in your hands at all! Your desire to save your friend is very admirable but it also sounds like it could be self-sabotaging to some extent.

“Rehab doesn’t work” is a blanket and not entirely true statement. There are a million different pathways to recovery; not every programme works for every person. Maybe try to explain this to them.

Beyond that, the best thing you can do for them right now is to disengage and remain distant. You don’t deserve to have their pains inflicted on you too.

P.S. I am speaking as a recovering addict. One of the things my recovery has taught me is how much of a burden being an addict is to other people. The thought of a relapse hurting my friends disturbs me. Your friend might resent you for turning them away, but when they do start recovery, they will not only understand why; they’ll appreciate it too.