Damaged screws aren't in themselves all that bad. Look for signs that the thing has been apart if you're concerned that BillyBob has been inside adjusting things.
If every screw on the gun is twisted, his further skill to take things apart couldn't have been much better.
It's usually easy to see if the things have been apart with some careful looking over.
Breaks in the old wood to metal fit and put back together again are a big clue. Scratches from prying with the one and only screwdriver, punch marks (nail) to drive pins out, etc.

How is the gun mechanicaly now? Is everything operating manualy as it should?,,Safety, triggers, top lever, etc.

You could almost make a living just fixing screw slots on quality guns. People just have to test them out with the one size fits all 'driver.

Many time nothings been disassembled past a simple forend or top tang screw being taken out. It doesn't lead to anything for the un-informed and they stop there,,but first they have to make sure all the screws are good 'n tight.

Don't immediately dismiss a gun just because the screw heads are damaged. Give it a closer look.