Can you afford a neg and bad DSRs?
I suppose he is in the USA. It will cost him at least $20 to return the disc with Delivery Confirmation. If you were not a helpful person, and he returned it without DC, you could win a Paypal dispute because he could not prove delivery.
Have you even received the payment yet, or is it still under the PP 21 day hold?
I'd tell him to send it back for a full refund. When it arrives, you can test it again. If it works then you can refund just his original payment . He will be mad.
If it doesn't work, then ethically you should refund the return shipping too, since the error presumabley was yours.
This is a silly question from a technodolt, but is used software like that Leopard actually saleable? I thought it could only be used on one machine or on several machines with the same owner.