As one of our rational members pointed out, it is most likely that in the end you will have to return the money for the freewheel.
The return would be at the buyer's cost. One can certainly hope to refuse it if; it is not returned in the condition sent. I do not know if you have to return the original cost of shipping.
This buyer will likely leave a bad feedback no matter what you do. Document what you can. Play by the rules and be able to live with yourself. Even when the other guy is in the wrong, it is better to be a good guy yourself. This does not mean that the remorseful buyer need not feel a bit of pain himself by incurring the costs of shipping. the buyer certainly knows what shop he went to for the take apart and the alleged mechanic. request that information. Contact them to verify his claim. That request in itself may cause him to back off.
I feel for you but I just can't reach you. Out of curiosity; where is the buyer located? Perhaps a CR member can do a bit of leg work for you. Good luck and happy trails.
john strizek
sacramento california
in the USA