"Any ideas on what happened"
Well, some guesses:
If you're hoping it will go away on it's own, you'd want this cause to be it, especially if alcohol was involved and there was sustained pressure on the **nerve**:
https://chiropractorchandler.com/conditions/shoulder-pain/saturday-night-palsy-treatment-not-what-you-think/
That's prbly the most common.
Or a worse situation: lack of blood flow from compression of the **artery** can cause "ischemic neuropathy". That is, a nerve can be damaged in a few hours from lack of oxygen.
"...or what he can do?"
or if the problem is persisting because a **clot** has formed in an artery, then aspirin is a possibility. But merely doing self treatment is taking a chance, as you must have figured. Why would he be susceptible to clots? Overweight, diabetes, vax booster are possibilities.
I wouldn't raise the arm any more, for sure. Mabe a little blood flow is getting through, but that reduces when the arm is raised. So you don't want further episodes of that ischemia.
A rare possibility is "thoracic outlet syndrome" in which a person has an extra rib (high up) that compresses the artery to one arm - that's prbly more likely if he slept with his arm up near his head.
A big question: are the fingers white or worse yet getting black?
Good luck.