If you leave it in the sun for a short period of time, it will mainly just kill the mildew. Some of the mildew, probably not all of it. If you leave it in the sun for a long period of time, it will fade the whole back of the card.
I live in New Orleans, and some of my collection got wet due to Hurricane Ida. I tried this strategy of leaving the cards in a card box, loosely packed, sitting on the dashboard of my car.
The sun and the heat did kill a lot of the mold and mildew. It made things better, but I wouldn't say it was a cure-all. His little spots of mildew on his card is much less severe than what I was trying to deal with, if he leaves the card in direct sunlight for a while, he might have a good amount of success. If not, he can just go back to the original suggestion, and pitch the card and buy another one.
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