The insurance company of the last owner owns it but theres probably a loop hole where she can keep it or the last owner can buy it from the insurance company for what they paid him for it.
Why would her insurance company own it? It was stolen BEFORE she bought the car so she has no right to it 'nor her insurance company. Her company has no claim against the car, it wasn't stolen under her possesion. She has no choice but to give the car back (she was trying to sell it anyhow) as the original owner still owns it. He never got a payout from his insurance when it was stolen back in 1970. So his insurance doesn't own it now, or whatever company he was with back then.
If your vehicle is stolen and you either don't have theft or decline the payment for the theft from your insurance company then you still have claim to the vehicle if it's ever found. Your insurance company won't have anything to do with it.
You could also take the payout from the theft but also ask to be notified when (if) the vehicle is found and have first chance to buy it back before it goes to an insurance auction. Same as buying back a vehicle after it's totaled. In this case the insurance company WILL own it after the vehicle is found and will continue to own it until either you buy it back or pass on it and it's sold at auction.