Considering the circumstances of the oil pump dieing, I would be asking the tuner to sort out re coating your pistons and head. If any damage was done to the rear wheel of the turbo that too.
That much seems fair, a full rebuild does seem a touch rich when the bottom end would need redoing at no fault of his own. It is right to say he is a total goose and should pay to rebuild your motor, but you would have had to do the bottom end either way, so why fight that. More hassle than its worth.
I would definitely proposition him to pay for the ceramic works a new turbo wheel if need be and put the matter to rest. Offer for him not to worry about finding the new motor as that much is what he is responsible for and the rest was out of his control to an extent. If he tries to deny liability in total, remind him he is offering to buy you a new bottom end and that by any legal standard is him fulfilling his obligation of taking responsibility (this is how we screw insurance companies that will say we hold you at fault but willing to let it slide).
I feel for you man, must suck. GL