Roofman1 is on target - remotely possible hand-cleaning with a paint remover on a brush and rag would work, but any liquid remover that gets on the shingles is likely to damage the surface and the tar - certainly if any soaks in.
Alos - time to attack this wouldhave been in first few hours after painting when it could have been easily taken off.
If I made this mistake and homeowner was amenable to a decent but not perfect repair job, I would look (by testing and chemistry) for a paint remover that would take the paint off but not attack asphalt (or whatever your roof is made of), then use a very stiff brush wetted slightly with paint remover and work from bottom to top, brushing and dabbing with paper towels to remove the paint then immediately flooding that area with water so the solvent could not attack the shingles significantly, working up the roof. Then "dry brush" matching oil based paint onto the shingles as needed to match color.
If wood roof, then should be possible to GENTLY sand it off with a handheld small power sander - then reseal/restain those shakes or shingles if sealed or stained.
If tar roof, then a sprayable tar repair material should do the job - comes in spray cans just for this sort of surficial repair.
If on undersides of shingles where they overhang, I would just let him repaint by trim brush with an oil based paint that matches the other undersides, or if wood maybe let him paint the undersides of the overhangs all along in a decent dark or matching color.
"Right" way if on top of the shingles and quite noticeable is to replace those shingles.
No "typical" compensation - it would normally be what it costs to replace the shingles.
Not to be hard on him, but this is the sort of thing the contrator's insurance policy is for - if he will not have the shingles replaced at his cost by a roofing contractor you accept, then maybe time to file a claim with his insurance company. They may make you an offer in cash to avoid paying for replacing the shingles - I would guess you would be offered about $250-500 assuming he oversprayed pretty much all along the edges for a foot or two wide.
I would talk to him about what HE intendsto do to make it right, subject to your approval - after all, it is HIS problem to correct, not yours.