Get her on the pill straight away. An 18 year old is definately going to be after one thing IMO.
There is not a lot you can do apart from insist that you pick her up from any parties etc. especially if he has a car. Boys at that age have a tendancy to drink and drive and 15 year old girls think it is cool.
He must be a bit socially maladjusted to fancy such a young girl anyway which is what would bother me. When I was in sixth form only the very direst of nerds dated high school girls. Her friends will all think that she is ultra cool though which is hard to fight against.
I wouldn't hold his background against him until you know better. Although his father does sound unusually fertile and lacking in contraceptive knowledge which may be a bit worrying.