Now he does ? Did you supply the clips he has ? They may be after market or worn out and the notches won't hold.

If it's the gun , check the clip latch spring and the clip latch hook that grabs the clip. If OK , mike the "hump" on the bullet guide , the part of it that sets against the bbl. New should be .079 or so ( seen them to .082 ) , reject is .076 ( mine was down to .073 when replaced , same problem). If still nothing , you are going where I've never been. Check the follower arm for wear and straightness , check the mainspring fork for the same. ( these would affect the op-rod latch which , as you know , has an arm that works the clip latch).
HTH , Chris