If the dad is a friend, I would assume he's a good guy. You should first tell the boy what he needs to know about sex and safety. Then tell him to talk to his dad and be open about sex. And later ask him if he's talked to his dad. If you give him a reasonable amount of time, say a week or two, and he still hasn't talked to his dad, then yes, tell him. The boy may be pissed but he'll hear what he needs to hear, and a boy's dad needs to know everything. Been there, done that. This assumes the dad isn't a prick.