The veering when you chop the throttle is normal in any single screw application, what is happening is the sudden reverse of the propeller torque. Under power the boat leans one way when you chop the throttle it will lean the other way, it is far more magnified on deep V's. Now a wild guess he has a cleaver or a cleaver style prop, I tested two different cleavers on my 22 and found them unstable. I could make the boat lurch to one side at high speed just running the trim in and out, switching to a round ear prop eliminated that. George had the same issue with his Minx, he gave his misses and friends a near heart attack one day. He like me dumped the cleaver! Many others have had success running them, note the switch from a same pitch cleaver to a round ear gave me 2mph. I am willing to bet drive height is not you guys issue, I have done quite a bit of fooling around with height on O/B's. A deep set lower contributes to chine walk, raising it helps to eliminate it, don't lower the drive! Borrow some props for testing, his current pitch sounds about right I think he just needs a different configuration prop. It takes some time to get a boat setup up with the right prop, and no two boats are alike.
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.