I’d never touch another person’s instrument of any description without their permission first, especially guitars. Even upright pianos that have lived in a hallway for 20yrs, still ask before tinkling.
And as an owner of precious things, when I’ve had others over to play I’ve always said “you are playing this and this only!” while brandishing a wooden school ruler to smash wondering fingers!
Well maybe that last bit about the ruler never happened but I did have a rep of being “stingy” with what I’d allow others to play and I believe it was with good reason after someone split a beer on my MS-20 (down the back but still, they split a beer on it!).