Steady on.
I could see it quite plausible to replace the mech and chain along with the hanger.
My view would be that leaving a slightly bent mech hanger or kinked chain could quite easily lead to the same happening again.
Thus by replacing the lot it’s minimal risk to the store. I’m sure the customer had other options I.e. a LBS.
Labour still costs money. I would rather sell an £8.99 chain than fart fanny around with splitting oily chains and inserting a new split link etc.
Halfords serve a purpose, and I’m sure they were well chuffed with your help!