Does bending drywall weaken it?
Gypsum (drywall) panels get most of their strength from the paper face that wraps them. Generally speaking, if this is intact the sheet maintains its integrity. Without the paper, the gypsum inside is soft and brittle.

However, severe bending cracks the gypsum core at frequent intervals. This reduces overall stiffness in the sheet, which can mean a wall or ceiling that feels less stout. Occasional cracks in the core don't mean much if they're not closer than the distance between framing members, for example. 

Any sheet that has been bent beyond about 45 degrees over say half its length could be considered compromised. Any sheet that's been bent 90 degrees, or that has been bent to a lesser angle but sharply, shouldn't be used, in my opinion, or should be used in less critical areas. 

I'd raise the issue with your contractor and have a closer look. The lumber yard should take responsibility for any damaged product.