What's to say they didn't crack the socket on the previous job anyway? Sockets brake all the time, that's all covered under the labour costs, just like their electricity, rates etc. Next they will be trying to charge for the use of the lights in the workshop.
Ask them if it's a snap on one if it is tell em to shove it they replace them foc if not tell them to buy one !!! Break mine all the time I am a tyre fitter and would never ever dream of charging my customer . Even if the nut that over tightened it before causes the problem