Hairdressers got dye on my top

I’m looking for some advice regarding an issue I recently had at a hairdresser in London.

I had my hair dyed recently, and unfortunately, some of the hair dye got onto my jumper and has left a permanent stain. I have washed the jumper and tried using Vanish, but the stain is still there.

I contacted the hairdresser to make them aware of the situation, and they have asked me to provide the receipt for the jumper. Unfortunately, I no longer have the receipt as I purchased it some time ago. The jumper is also no longer available to purchase from ASOS, so I’m unsure what I can provide instead as proof of purchase or value.

The hairdresser has already been quite defensive and reluctant to accept responsibility, so I’m concerned that simply explaining that I no longer have the receipt may not be well received.

Has anyone experienced something similar?
Any advice would be greatly appreciated.
Thank you!