Include all three parties in the discussion. The goal should be to make everyone satisfied. The seller might negotiate with the marina to accept less than the full amount of rent. The marina might accept less if they get it out of the sale proceeds. The seller might be satisfied to receive most of the purchase price. If your friend gets the boat he expects for a price he considers reasonable, he should be satisfied. Your friend should proceed with caution in dealing with any person who is in financial distress.