What do you mean "loosened the brakes"? Did he flip the quick release on the caliper or did he loosen the cable. In either case, this a good learning experience. You should check your bike for things like that before every ride. My suspicion is that the release was open. They could have left it open or you could have accidentally done it. It would be hard to prove either way. I would just be happy you didn’t get hurt.

Every new bike gets a scratch eventually. They build character.