Hi, I would think the stain has penetrated the surface of the PVC. There are two products that may help, one is used by window installers, and is intended for the purpose, Siroflex Solvent Cleaner. The other would be Cif cleaner, as you suggested, it's very similar to the purpose made PVC restorers on the market. Try a small area and see how it goes. Do not try Brillo.
It's worth a try, but I suspect your neighbour is going to have to pay out !
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