There should not be an accent on the pronunciation of "cena." It sounds like she says cena' and not cena.
Why the sentence 'He learns after dinner.' is not accepted? Is that a different meaning?
Although the literal translation of "lui studia dopo cena" is indeed "he is studying after dinner" it can also be correctly translated as "he will study after dinner", depending on the context. Since no context was given in the Duolingo test, the latter translation should not be marked as incorrect.
I'm getting the answer already printed in English when that is the question?