Sounds very unnatural - are we trying to say "He is eating all the fruits", as in all the fruit here on the table? Or "He eats all fruits", as in there are no types of fruit that we would not eat?
I was just wondering the same - because the second seems more likely to me.
"She's eating all the fruit." Fruit is uncountable, at least in England.
With "the" it could mean that you are dismayed that this guy is finishing off the entire bowl of fruits that was supposed to be shared by everyone.
as with many of these exercises '-'s' is used to abbreviate 'is' and this is best not done in a language course