"Why don't you tell her the truth right away?"
Translation:Pourquoi tu ne lui dis pas la vérité tout de suite ?
It's because of direct objects and indirect objects, or accusative and dative.
Lui/Elle is when it's a direct object. "I will thank her"="Je la remercierai".
Lui/Lui is when it's an indirect object. "I gave her a gift"="Je lui ai donné un cadeau". And here "a gift" is a direct object, for example.
Here lui is an indirect object pronoun that works for both masculine and feminine, and la would be wrong.
|Indir. obj. pronoun||Je lui parle||Je lui parle|
|Direct obj. pronoun||Je le vois||Je la vois|
|Stressed pronoun||C'est lui qui parle||C'est elle qui parle|
Parler à takes an indirect obj. pronoun.
Voir is followed by a direct object.