Before reading other responses:
The player's decklist does not match their actual deck. However, this seems like a pretty obvious clerical error, and there's no room for ambiguity that could allow a player to gain advantage. The IPG allows for:
In Limited events, the Head Judge may choose not to issue this penalty for incorrectly marked basic land
counts if they believe the correct land count is obvious.
I'm assuming “ half blue cards and half green cards” means that the decklist has about 50/50 green and blue nonland cards, and that the deck matches this. If this is the case, no infraction has been committed. Correct the decklist to match the deck, and remind the player to be more careful when filling out future decklists.
After reading other responses:
Mark's additional comment about letting the store owner know their actions were correct is a good point!