lets hope he hasn't got too many bolts to undo.
I've found the best way to loosen rusted bolts is to add penetrating oil. If you can, tap the head of the bolt to brake the seal the rust has made. With screws, it's the same thing. Use the correct size screwdriver, place it in the screw and tap it with hammer.
if all else fails, drill the bolt/screw out and use an esy-out tool. Most engineering shops stock them.