In reply to you post. A lot depends on the age and the mileage of the car you sold, which you have not stated but I think you did the right thing, had the clutch been slipping or had it gone within a short period of the customer owning then you would have had to replace it completely free but given the circumstances you have done right by your customer by offering a compromise by supplying the parts.
I think the customer (if he takes independent advice) , will accept your offer of goodwill as the clutch is worn, and as a car is full of parts that move and wear there has to be an acceptance of wear and tear from the buyer. If you are buying a 5 year old car don't expect a car with a 6 months clutch in it!
If he refuses to accept this and goes for the return unfortunately you have an extra owner on the V5C unless you can convince the customer to write a letter to send along with V5C to the DVLA explaining that the car had been registered in his name and address in error, this may get DVLA to remove the details, but more often than not returned cars will have an extra owner!
Best of luck
Jim