Decks are made of steel, what the hell did he run over that "tore up" the deck but did not damage the blades?
He offered you half of the $ for a new mower, which is less than it would cost him to pay for repairing what would still be your POS mower. It would have a new deck, and the same worn out chassis, suspension, steering components, tires, seat, blades, bearings, battery, belts, etc, and of course that poorly maintained engine. Seems like a damn nice offer on his behalf and IMHO you're a fool if you don't take advantage of his generosity.
Pics of your mower & damage?