Translation:Since then her behavior has been excellent.
Although present tense is used in Portuguese, in English we use the past tense here. "Since then, her behaviour has been excellent."
We can say "o comportamento dela tem sido excelente" (best option), or "o comportamento dela vem sendo excelente". (Portuguese present perfect does not mean the same as English present perfect)
That's present perfect being used. (An action that starts sometime in the past and continues to the present time.) Meets the requirements that it impacts the present and is still true.
Past tense would be "was excellent".