I don't know if I would buy a tub which has not been paid off. I suppose you could talk to the loan company, and pay them directly - keep in mind:
You MUST have proof that he has paid off the loan, or they could come looking to you to pay. The tub may not be the collateral for the loan: if he has used his home for the collateral, you are fine, but again, you need PROOF of that.
And, again, you might contact the loan company directly and buy it from them with the help of the tub owner.
Virtually all spa warranties do NOT transfer to a new owner.
HTH