I'm not sure if I could comfort them but I'd definitely spend time with them listening to their problems and if possible, trying to direct them to someone/something that could help with their issues. Happened quite a few times to me. Even people who are not my close friends sometimes open up about their problems to me and I try to listen to them. Of course, I cannot help them all the time because I'm not a trained therapist but I feel it helps them to get things off their chest.