well.. from my experience, id gotta to say that in general most guitarists are really uptight about this sort of thing. a good number of my friends play guitar and they bitch excessively whenever i fiddle with it.
unless your friend fiddling with your guitar is stupid or has been known to break lots of stuff, then i wouldnt worry about it that much. the strings are made of steel you know :p. but i can totally understand why youd be protective though. a guitar is expensive.
one time i did rape my friends guitar though. i leaned it against an amp and it fell on the pavement.