You're so right to be concerned. My junior year of college, I moved into a house with my 4 best girlfriends. By the end of the year, none of us was speaking to the others, which I regret to this day. I'm still mildly in touch with one, 20 years later, but haven't a clue what happened to the rest of them, and I miss that.

You're a good friend and have done what you can do. It's going to be hard to sit by and watch, I know. But as Wants said, resist the temptation to say "I told you so."