Insurance write off?

[Little Lofty]

A good friend of mine bought a car in the summer, within seven days a young lady rear ended him at a junction. The car was bought for £6400 in a private sale and the damage was estimated at £5k. He was using a claims company and they originally told him it would be wrote off, but then told him it was being repaired. The car was indeed repaired and he was happy to get it back. Fast forward a couple of months and for various reasons he decided to sell the car, to his horror he has discovered it’s a CAT S, not from previous damage but from his accident. Many phone calls to his insurance company, the claims company and the third parties insurers have got him to a point where it looks like it has been removed from some registers but not all, in particular it is still showing on Experian as CAT S. He has found out the third parties insurer did write it off and had sent him a cheque via the claims company, who returned it as the car was then undergoing repairs. One dealer is offering him £3k He’s hit a dead end with the insurers who keep telling him it’s clear, whilst the dealer he is trying to buy a car off is telling him it’s still listed as a CAT S. Does anyone have any ideas how he can proceed. Thanks