"I gave her back her computer."
Translation:Je lui ai rendu son ordinateur.
Because this is an indirect object: rendre quelque chose à quelqu'un:
Je lui (to him/her) ai rendu
Seems like this could also mean "I gave him back his computer". Is this one of those situational situations?
Since "lui" is the indirect pronoun for "il" and "elle" when the preposition is "à", you are right, "je lui ai rendu son ordinateur" can be translated to your masculine version as well.
When you get the French sentence for back-translation to English, you will, therefore, have a choice between the feminine and masculine versions.
not fully understanding this, could someone explain a bit more why 'je l'ai rendue son ordinatuer' is not the correct answer?