IRS will most likely make him pay the entire tax because he received the money, a taxable event. He can then split it with his buddy, with whom he has a binding agreement.
Of course, The question is whether the payment to his friend is a gift or payment of a contractual obligation. There are different tax implications associated with each. If it’s deemed a contractual obligation, he would not only be entitled to an income tax deduction for the amount of the payment, but the payment would not reduce his unified credit for estate and gift tax purposes. Depending on the circumstances, the IRS might not like that outcome and could challenge his assertion that he was subject to a contractual obligation.