unable to swim

Question
I have a friend and he knows how to swim he just can't. He can swim ten or fifteen feet and then begins to sink. Why is this? He tries and tries but everytime he will sink.

Answer
That is a great question. Has he had the opportunity to work with a kick board on endurance? How is his stroke technique? Perhaps that is the best place to start. Sorry about adding a question to a question but if you get a moment and could answer those I may be able to help.