The unnamed trainer is a character appearing in Pokémon: DP Galactic Battles.
Pokémon the Series: Diamond and Pearl
This trainer went to see Professor Oak and asked him if he could take a look at his Tyranitar since it doesn't listen to any of his commands. James, disguised as Professor Oak, went to see Tyranitar first but he failed and then the real Professor Oak managed to solve the problem. The trainer then thanked Professor Oak for helping him.