Cutting it wont hurt anything. Its just a steel line that can easily be fixed with a compression fitting. Thats the only way he is going to be able to get the fuel filter out of the car for him to have enough leverage to get the pipe fitting out of the fuel filter. Trust me, I've had to do this before on my car and a couple customers. Plus, if you ever repace the fuel filter again, just take off one side of the compression fitting and you can take the fuel filter out of the car and get at the flare line fitting that is threaded into the fuel filter.
If he keeps wrenching on the fitting inside the car, he is most likely going to kink the fuel line and have to cut it anyways.