How did buyer request a refund? Buyer should have opened a not received case as global are responsible, your proof of delivery to global is all you need to protect against loss in post.
If they did open a case which I suspect they did not, then you should respond with jtracking details.
There is very little point in using Global if you fail to use the benefits of the system. Having refunded, I have no idea whether Global will repay you as that is not now it works.