I think you're both technically right-
Using the plural with an s, "fruits", and saying "a banana and an orange are fruits" reinforces the idea that there is a variety, that while both are fruit, they are different kinds.
However, native English speakers rarely use "fruits", except maybe:
-where we need to refer to all fruit: "the fruits and veggies are in aisle 3" (and even here we can say "the fruit and veggies")
-or idiomatically: "the fruits of his labor"
-or to point out specific types of fruit: "My favorite fruits are bananas and oranges."
-or if three or more are listed: "strawberries, apples, and grapes are his favorite fruits".
Even then, we usually say "My favorite kinds of fruit are bananas and oranges" or "strawberries, apples, and grapes are his favorite types of fruit."
If there were more than two types listed here, I think it wouldn't feel so uncomfortable. But for the ease of the exercise and a more natural-sounding English translation, I'm inclined to go with "fruit". "A banana and an orange are fruit" or "a banana and an orange are types/kinds of fruit."
'In botanical terminology, a berry is a fleshy fruit without a stone produced from a single flower containing one ovary. Berries so defined include grapes, currants, and tomatoes, as well as cucumbers, eggplants (aubergines) and bananas, but exclude certain fruits commonly called berries, such as strawberries and raspberries.'
Really, of course, we wouldnt say either. It would "bananas and oranges are fruit/fruits"
I have no idea about the actual grammatical reasoning, but I would say . . . A banana and an orange are 'fruit'. (not fruits)
From what I know, if they are different types of fruit, then it's okay to call them 'fruits'. Although maybe indeed 'fruit' would still be used more often.