I think Tartan may be on to something as well, as this was my first thought - soap residue or a chemical reaction... the friend washed it to rehydrate it and clean it, and she did follow the manufacturer's directions. She has been in contact with the manufacturer and they're more or less accusing her of trying to scam them using an old blade and saying it's rusted because it's old. She's very frustrated and will likely buy another brand of mat in the future. Of course, per the rules here I can not and will not name the manufacturer, especially since my info is second hand....
Thanks for the thoughts and ideas....