friend had car insured with 6yrs NCB , upon renewal he changed car and insurance co , the new insurance company are requesting proof of his NCB on his new car(he didn't tell them about old car) , but regardless of this i find the car he was insured on for NCB completely irrelevant , in 20years of proof of NCB no insurer has ever requested details of the car the NCB was earned on. All that matters is that he has not made any claims in 6 years - whether they were earned in a battered lada or a ferrari is irrelevant surely?
i should not think they are asking for proof of what his old car was,just proof he has earned the 6 yrs ncb,which his old company will gladly provide in written,if he asks for it. i think you might be a bit confused.
The reason they would query this is that NCB can only be used on one insurance policy at a time, so it would look to the company like your friend is trying to use the NCB for a second car, as the renewal notice (the proof of NCB) will show a different registration number to the one being insured. All they would need to do is inform the company that the car was changed at renewal time and all should be fine.