Don't serve them anything. In that case, this poor starving stranger will ask to your friends chowing down, "Can I have some of those lamb chops?" Then you know to serve them lamb chops. Plus, if you don't know this person, why on earth would you invite them for dinner?
Their flesh would be a good start. Although since you don't know them it may be a good idea ask about their medical history to say how safe that is. If you can tell their ethnicity, German brains supposedly taste good.