A friend of mine is in the process of buying a house with someone they're dating who seems very flaky/noncomittal about the whole thing and it sounds like a disaster waiting to happen in my opinion.
Tell your friend not to do it. The absolute worst psychological stuff I've seen happen with my friends is a break up. And the cohabitators had it 10 times worse. My view is once you get to that stage just put a ring on it. If you are that unsure have your separate places until you are sure. Morality aside it's just good advice. It is so much easier to walk away from a relationship when you don't have to move out and split stuff. Most relationships are over long before they are over. You really don't want to deal with some psycho and/or depressed girl when you are trying to move on with your life. Or you don't want to deal with the jealousy when you come home and you just know she was out doing the nasty with some other dude.
I don't agree with this based on my experience, but perhaps it is a stage of life thing.