First to avoid any future complications you will need a contract that he is willing to sign.
Its always better to pay a person even if they are friends this way the project doesn't get put on the back burner in favor of paid projects.
As many people have stated rates are all over the place. I think a fair price would be $500.00 to 1000.00. It really depends on how much work he has to put in on re touching the images. I know a few photographers who will spend literally hours on each image to make it just right.
