Well if he's 17, he could be off living in a squat or anywhere so I don't see that the parents can expect much control or supervision from his grandparents. As for the weekend partying well I wouldn't allow it in my house but what he does elsewhere wouldn't be up to me or anyone else..... I do think that to avoid problems, it would be as well for the grandparents to spell out to the boy and to his parents what they can expect and what they can't and what the house rules are and the consequences of breaking them. Personally I don't want a lodger, related or not.