You can at least obtain a record of the buyer's last download attempt timestamp and IP address from your E-junkie Transaction Log, which could help in cases where the buyer claims a fraudulent payment due to unauthorized access to their PayPal account, if PayPal can match that download IP to other, undisputed payment from the same account. If the Download Info column in your log is blank for that sale, and you hadn't manually reactivated their link yet, that would confirm the buyer never received or used their link, in which case (if the payment dispute is still open) you could try helping the buyer -- e.g., by sending them a personal email with their link (just right-click on their Transaction ID in your log and copy that link to send them).
Unfortunately, there is no technical way possible to prove that any browser-based download attempt ever proceeded to completion, nor that any completed download resulted in a valid/uncorrupt file, let alone that any complete and valid download produced a file that contained whatever the buyer expected to receive for their purchase. PayPal's business model depends on utmost buyer confidence, so they tend to give buyers the benefit of every doubt, every time, and thus these sorts of reversals are just a cost of doing business in selling digital goods online. You may wish to post a clear "all sales final, no refunds" policy on your sales page(s), but even that won't help with fraudulent-payment claims where PayPal cannot conclusively determine the account owner made the disputed purchase themselves.
You may be interested in some other sellers' responses and suggestions in this related thread:
http://www.e-junkie.com/bb/topic/4134
