He's a parent, she's his child living in his house. IMO, he has every right to go through her stuff if he so chooses. I think he acted like a prick by doing it, but that doesn't mean he can't figure out what's going on under his roof. What if he had done that and found heroin, would your reactions be any different?

I'm a parent with children living in my house. I have every right and responsibility to go through their stuff if I feel the need. I'll reserve the right regardless of their age, as long as I'm responsible for the house. If they have their own place, it's a completely different ballgame.