There's a couple of reasons I know of that can cause redness on the shell.. one is extremely serious the other isn't.

The serious problem is septicemia aka infection in the bloodstream which would need to be aggressively treated by a vet ASAP or the turtle will die.

The other reason I've seen before is reddish colored basking rocks that rubbed off on the shell & made it LOOK like they were sick. They just needed their shells washed with a soft brush.
Maybe someone else can pop on & come up with an answer.
Good luck to your friend.
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PHRatz

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