All you people saying tough luck must not remember what its like being 18. If someone does this for him it (without sounding corny) will touch his life forever. (assuming he cant get the loan) it will give him an opportunity that otherwise he would never have. He will now have equity for the first time which later may go towards a house or maybe even collateral towards a bigger loan.
Im all for opportunity and risk taking… If I was making 50k a year or had money banked that was just going to be sitting, I would just slap a 20% rate on his head and swing away.