Её яблоко can mean a third person's apple. One of my teachers used to say that in a case like this, we are supposed to use своё/свой/свои. But as far as I know, it is not a strict rule. Sometimes it is permissible to say её, although a better style would be to say своё. Hope it makes sense.

"Она ест её яблоко" indica que ella come la manzana de alguna otra chica o mujer. "Oна ест своё яблоко" indica que ella come su propria manzana. Para todos los detalles, véase: http://clasederuso.blogspot.com/2010/03/pronombre-posesivo.html

Она ест её яблоко — неправильное предложение.

"она ест её яблоко" - means third person's apple. But this is a rare construction. For example: - Я дала яблоко Тане, почему собака ест ее (Тани) яблоко? "она ест своё яблоко" - it's not necessary to stress a posession in russian language in this case. It sounds awkward. The right phrase would be just "она ест яблоко"

There's no difference.