...but I wouldn't tell him. I'd let the publishers handle that when it gets to that stage (which it may never), and be a supportive friend at that point. Of course, some really bad writing gets published too, so you never know I just wouldn't want to be the one to crush his dreams. I have a comparable situation with someone I'm quite close to as well, who is writing a book based on a very personal story and using it as a means of catharsis to deal with some heavy stuff. But it's awful. I have read various parts of it and done my best to talk about the things I did like about it, how creative it is and how impressed I am that he's persisted with it and really looks like he's going to finish it, that sort of thing. Things that are 100% true but are not "this is good" or "this is going to make you a gazillionaire" But I've also been working to try to make sure he has realistic expectations, as paradoxical as that sounds. So I tell him that it could be excellent but if there's already a glut of this type of book nobody will pick it up, or the timing could be wrong, or any other of a thousand things that keeps books from being published.

For now I'd make some minor suggestions about things he could flesh out or tweak (making sure to also point out what you DO like about it). So you're kind of doing what he asked, you're just not being completely truthful about it. In this case I think that's what's called for.