Shouldn't be more than a tenner for a windscreen repair via his insurer.
Tell him to get his insurance co's approved repairer to mend the chip (not a replacement screen - shouldn't need it from what we see in the picture ) and for him to send you a VAT receipt for a goodwill refund. Chances are he won't bother. However, from the wording of the Email, I suspect he probably will. Recorded delivery. CC'd to your local Trading Standards. And the AA, the local paper, TripAdvisor, That's Life and Rogue Traders c/o the BBC via FaceSpace.
If he insists it's £75 for a repair (which it won't be), or that he wants a new screen, I would suggest he supplies you with a copy of the insurance schedule as proof of costs. Then call Autoglass or whoever on his behalf and make the arrangements for them to repair it and send you the bill.
Or call his bluff and say it wasn't there when it left you.
Or tell him you've ceased trading.
Maybe not that.