You are doing a wonderful thing. I am in the same situation and have had "House Guests" for over two months now. It's been nice on some levels and not so great on others. I think the top thing on my mind as far as advice goes is this. Make sure they give you the privacy you need to continue to live your life normally. In a small house that is difficult, but you need a space that is your own and off limits to them, a place you can go to to sit and relax or read and just be yourself.
One of the people staying here is my sons ex girlfriend. She had a great job and just quit last month because she was angry with her boss. Now she sleeps all day and contributes nothing. I'm giving her a few more days to get her act together before I give her the "Get a job or get out" talk. I don't want to set her on the streets at all and will do everything I can to help her, but you do have eventually get tough to get them motivated.
Sorry I think I hijacked your thread...I just know what it's like to take people in though.
I hope your friend and her son help around the house and do try to get their lives together...it becomes a difficult situation if they don't. It feels good to know you helped someone out though.
Good luck!!