"The Solution" is right about having to prove harm was caused ... and that is evident in that the scholarship was withdrawn.
You can't prove that something isn't plagiarized, you can only prove that something is plagiarized. The whole "impossible to prove a negative" thing.
You would have to question the defendant in the case and put the onus on him to prove his claim, thus proving he defamed his accuser ... or proving that the accuser did in fact plagiarize his essay.
The proof is all on the Claimant. You have to first prove that your client was "harmed" by the defendants actions. (which having his reward withdrawn would be considered harmful) That is the easy part.
Next you have to prove the claims of Plagiarizm are not true. That he did NOT Plagiarize. If, Jamie in fact stole Drew's work, it is Not defamation of character, because what he said was true.