In the Mountains

You are head judging a Guilds of Ravnica Sealed PPTQ with a store owner (who is L0) as a helper, who is doing deck checks for the first time. You’re deck checking, and your helper tells you the deck he is checking is about half blue cards and half black cards, but on the checklist player has registered 7 Mountains and 9 Forests. He is actually playing 7 Islands and 9 Swamps, and everything else is correct. He doesn’t know what to do and asks for your help.

What do you do?

Answer
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