I am assuming the order is in the resolution center (RC)? if not advise the buyer to move it into the RC and work it out there, if the buyer has amazing stats it may be an issue but it is impossible to determine the outcome given each case is considered on its own merits. If you are unable to work things out with the buyer it may require an admin to adjudicate on your behalf, if you have any concerns or questions you should contact vendor support and advise them of the situation, they can then check to see if this buyer has made similar claims in the past.