Verdict = SOL
There is no way you can prove that its not her fault. As stated they can bring up salt, maintenance, cleaning with abrasive materials, etc. Unless you have months worth of footage showing how and when you washed it, where the car was stored, and where it was driven you should just give up. If they do replace them it will be 100% courtesy.
Same thing happened with my edge. Not sure what the last owner or the dealership did but when I went to wash the car my microfiber towel made the wheels started flaking all over the place. Ford laughed at me and pointed out the as is part of the purchase agreement. All I could do was show them 5 paid in full GM cars over the last 10 years and say this is $140K in profits your company will never see... not that they even cared.