Difficult question. Errors and damage can happen. An argument is of course not a good approach for finding a solution.
Depending on the year of construction, the car is 5 to 10 years old. It may also be that the heating mats are glued to the cover or to the seat cushion. This makes removal difficult. In addition, age and constant use (of the seat) can also have a negative impact on the robustness of the heating mat.
Sounds stupid, but it's positive that the workshop doesn't just hide the damage in the hope that you won't use the seat heater until after the summer.