To overcome the fear we have to face it right
show him videos of swimming 
make him to get interested in swimming
practice swimming daily
teach him the way he is comfortable

you have to teach your friend that water is not going to harm him and encourage him to swim.
well first of all let him go to the swimming practice for many days let him, so that he would  be prepared for all the over coming of the fear of water.
you can council your friend so that he can over come his fear.

hope so my answer is useful........^_^  !!!!