You’ve refused to pay your bill for a meal in a lopsided split. Were you right?

Your son went to dinner with four friends. One of the boys paid the bill...

Your son went to dinner with four friends. One of the boys paid the bill. Later that week, he asked your son and another boy for their share, but he did not ask the other two boys. Often those boys are exempt from paying because of lack of funds. Your son said he would pay only if the other two did. But mustn’t he pay what he owes, no matter what they do?