I’m sorry to hear that Tabitha is up to something so gross. This unfortunate behavior does have a name—coprophagia. If I could twitch my nose and make her stop I would.
Mother cats sometimes eat their kittens’ feces to keep their environment sanitary, but you didn’t mention any kittens in your message. While it isn’t generally harmful, this eating habit can lead to more disgusting outcomes such as vomiting and diarrhea, which in turn could lead to malnutrition or dehydration.
Tabitha’s vet can help your friend figure out if the root of the problem is behavioral (most likely) or medical. Bored cats can benefit from increased playtime and access to climbing structures. A hungry cat might need larger meals or smaller portions distributed more frequently. Medical causes might include a nutritional deficiency (necessitating a restructured diet), parasites or an underlying disease (thyroid disease or anemia, for example).
Of course it should go without saying that litter box poop removal should be done as quickly as possible in Tabitha’s household.