Most users often conflate deleting proofs vs documents. Is it possible that your user simply deleted a file that had a proof generated on it? if so, your admin might be able to retrieve it from the recycle bin.
Edited to add: after further offline conversation it appears that the proof was deleted as a version and retrieved through the Proof side of the house (aka ProofHQ). With this in mind, I don't know any other way to propagate it back into Workfront other than downloading it from ProofHQ as a marked up PDF and uploading it back to Workfront.
