I did the same as WTBH...I would confront XAH in the hopes of getting him to "realize" that he was out of control and that he needed help (from me) to change his behaviour. Unless you are able to speak with detachment, knowing that it'll probably cause a backlash of denial/anger on his part, simply say "I know you're using again" and walk away without any expectation...then go ahead.
IMO, there's no point in confronting seeing as you've already made up your mind that you don't enjoy living on a rollercoaster.