Could I add to haber le avisado: a ella?

Hola Emanuel

There is a typo in this sentence, actually. The "le" should say "la" as "avisar [a alguien] is a transitive verb and "la" is a direct object pronoun. You can repeat the direct object saying:

The repetition of the object is simply there simply for emphasis or clarification. You could also omit it and just say:

If the translation was "You should have warned him (not her)", then you'd need direct object pronoun "lo":

I hope this helps

Saludos

Inma