"She is talking to him."
Translation:Elle lui parle.
"Le" (and "la") are direct object pronouns. Indirect object pronouns are different for 3rd person singular and 3rd person plural. See my note above.
We recognize "to him" as the indirect object of the verb. In French, there is a different set of object pronouns for direct objects and indirect objects:
- Direct object pronouns: me, te, le/la, nous, vous, les
- Indirect object pronouns: me, te, lui, nous, vous, leur
"Elle parle de lui" = "She is talking about him" or "She talks about him"
That would be fine for "she is talking to them" but this sentence is "she is talking to him". The indirect object pronoun for "him" is "lui".