Native English speaker here. In my opinion, saying "She only eats fruit" makes sense because it doesn't specify what kind of fruit per se. But conversely "She only eats vegetables" makes sense but "She only eats vegetable" doesn't flow right. Anyways there is no hard or fast rules in English since like all languages, it is a dynamic form since it is susceptible to change.
Can someone tell me why fruit is singular here? Because english sentence doesn't seems natural to me at all. She can either eat only fruits or just one kind of fruit, that would "eats only one fruit", though I don't think portuguese sentence has that meaning.
For us, to say "ela só come frutas/frutas" has the same meaning (she only eats fruits) but the sentence duolingo used is the way most people say.
Maybe the English sentence has been changed since you posted this, but right now it's "she only eats fruit," which is perfectly natural.
I wouldn't. "She only eats fruit," sounds better to me than "She only eats fruits," but I think either is correct.
Basically, you'd be using the word "fruit" as a category rather than a countable noun if you say "she only eats fruit". That's why it's a natural thing to say. Although it's equally fine to say "she only eats fruits", but then it's a countable noun. Not really any difference in meaning just two ways of putting it.
If you say "fruits," it most likely means different types of fruits, a variety like apples, bananas, oranges, lychees, etc. If you say "fruit," it means fruits in general, but it can be all of one type of fruit, like Rome apples, Fuji apples, McIntosh apples, etc.
Fruit is countable or uncountable, depending on where you are from. In some places it is both.
«Ela é a única que está comendo fruta.» or «Ela é a única que come fruta.», although this second one sounds like no one else likes to eat fruit.
This leaves me curious as to whether "Só ela come fruta" would also have the meaning that marcobe25 asked about, and if it would be a "natural" way to express that. My understanding is that só can be either adj or adv, so... ?
What about "She alone eats fruit." How would that translate? (As in, no one else is eating fruit). Can "so" be used in this context?