It does not take a professional to operate a basic gas pump. The retainer is what was faulty and a quick look, which can be reasonably expected of any John Q, before use, is what is the issue. I don't see the pump as "faulty". He could have easily seen upon a quick glance that the retainer was broken. That is not too much to ask of anyone able to drive a vehicle.
Also, nozzles have a built in back flow valve, retainer or not, which stops the pump when it gets backflow.
This is his fault. Period.