"We are going to reply to her."
Translation:Nous allons lui répondre.
"Lui" is the indirect object pronoun for both genders. "Leur" is the plural form.
It's proper French to use the indirect object pronoun (that goes before the verb) if the object you want to use is a stressed pronoun itself. For example, we can say "nous allons répondre à ma mère", but not "nous allons répondre à elle". Instead, if we want to replace "ma mère" with a pronoun, we say "nous allons lui répondre".
The object pronoun of a verb must be placed immediately before that verb. "Lui" must be placed immediately before "répondre".
No, because "répondre" takes an indirect object, and "le" is not an indirect object. The full construction is "répondre à quelqu'un".
It's similar to saying "talk to her". We don't say "talk her".