Test the customers resolve explain that there was no dammage to the carpet when you left and explain that you will contest any action she wants to take. If you give in to these people they will do it to some one else.
I once cleaned a carpet for a lady who was going on holiday and leaving her teenage son to over see the work, about 2 weeks later i got a phone call saying that we had torn the carpet in the middle of the room and what were we going to do about it, i explained that the carpet was fine when we left and she should have a word with her son. A couple of weeks past and i heard nothong untill a trading standards officer called, i welcomed him in, made him a coffee and proceeded to show him the equipment we use, and expalined that the lady had gone away leaving her son in charge. He agreed with us there was no way we were to blame and would recomend that the lady talk to her son about what had gone on while she was away!
Dont ignore the matter deal with it head on, theres always someone pulling a fast one.
John