My advice to him would be…
Take off a couple of months with no golf. Come back. Hit nothing but pitches for a week.
Week two, hit nothing but pull fade 7-irons from an extreme forward ball position.
Go play every day for 7 days where every shot, including tee balls, is a 9 window shot. Rotate them.
Then play 4 consecutive weeks on the PGA tour.
He needs to level out his shoulder turn, but he needs to dump all internal cues and just get back to shot making. Week two will work on the major mechanical issue without him thinking mechanics.