Depending on the model of single shot, the bolt head that connects the forearm to the barrel may be exposed. If your girlfriend shoots a number of shells one right after the other, the bolt head will get REALLY hot. I had this happen to a youngster when he was learning.
As far as ammo goes, look for "light" target loads in the 20. The 28 gauge is becoming popular again, so there may be someone at your range who has one and will ket her shoot it. It usually has less recoil.
shoot straight
C