Marmar, suggestion No 1....ask her to replace the damaged mat since she admitted she was the culprit. I think, had I done that I would have offered to pay for it. Quilting equipment is not cheap, or else why did she not have her own equipment? I would be a bit more than annoyed. She took partial responsibility. Let her buy a new mat and you can give her the damaged one to practice on for a while.
I hope you get a new mat. Fiskars will respond by e-mail quickly. they did for me.