Kind of hard to do with no video.

I would take a wile stab that he's turning it super early then maybe trying to save it at the bottom with lots of fingers.

If that's the case, have him trying leading into his release with the inside of his hand and holding it as long as possible.  If he is turning it early, he'll feel a ton of pressure on the inside part of his hand/arm.

Just a guess.

Bump

Sounds exactly like what I go through.. I slide way too early and then I have to plant which causes so many bad results like turning the ball way to early.. I could never get any consistency with my timing or my release.. After 20 years of trying to fix the problem, I pretty much just gave up. I've had many lessons with several different coaches and no one has been able to help me.. The most frustrating part is that I know what I'm suppose to do but I can't get my body to do it.. I wish your friend good luck and I hope he can fix this..