Co-signing the apartment may be the solution you want, but is that the solution that they actually need?
Unless you are a mental health professional, it's difficult to assess what's going on with your friend. And IF you are a mental health professional, you wouldn't be able to ethically treat him because you just don't have the required distance needed to be neutral about his case.
You can't throw your own life and your credit rating into the volcano to save him.
You mentioned NA. Doesn't AA/NA strongly discourage sponsors from providing financial assistance to members? If that's the case, I'm sure there's a very good reason why.