OK, I'll take a stab at it.
Since your friend's brother-in-law bought the car with 8K miles on it, I am assuming that he has owned it for 35+ years. That speaks volumes, but not as many volumes as if there was real paperwork on the car. If we assume no paperwork exists, I would guess in the $40K neighborhood. If paperwork does exist, I'd say add $10K.
