But Xander never actually SAW Willow's crash in 'Wrecked', he only heard about it after, and, as we all know, you can't really, completely understand a situation when you only hear about it second and third hand. Xander never really ever 'gets' magic, he doesn't understand it or it's effect on someone. It's not until he sees Willow first hand at the end of the season that he 'gets' it, sees just how much magic has twisted and turned Willow and it's then that he understands just what he needs to do to make Willow stop - not appeal to the fact that using so much magic is dangerous to her, but to show her that using too much magic will kill the people she loves, Xander first and foremost.
Up until that point, Willow couldn't be reached like that, which is why she automatically, without pause, turns to magic when Tara is killed. Because it has never occurred to her that what she's doing hurts other people, despite a lot of evidence to the contrary, but it isn't until she's face to face with Xander on that Bluff that she understands it.
It's also one of the reasons why Willow is so reluctant to use her powers in S7, because going dark side means that she loses the ability to care about those she loves and she won't care if they get hurt in the crossfire.
But I have to say, despite all of this, I don't understand this...I don't know, need, I guess, to put them blame for Willow's failures on someone else. Willow messed up, she got herself addicted and, the same with some addicts in the real world, she doesn't listen to those who love her and try to help her, because she thinks she knows best, until it's too late and she almost dies and takes a lot of people with her.
Willow and Willow alone is responsible for her actions throughout S6. Just because Xander was too caught up in getting married and then trying to fix himself and his relationship, doesn't mean that he deserves any kind of blame and I don't think he should be assigned any.