"I know her."
"sapere" means to know how to do something. "Io so sciare" "Tu sai scrivere" "Lui sa l'italiano"
"conoscere" means to know somebody or something. "Io conosco un bravo dottore" "tu conosci mia sorella" "Lui conosce le buone maniere".
Sometimes both "sapere" and "conoscere" can be used, but not when we speak about "knowing people". In this case, only "conoscere" works.
Another possible translation would be "Io conosco lei"
Hope this helps someone in the future: Conoscere is a verb that has implicitly a meaning of "having experienced something", "having experience with". Sapere implies that there is knowledge of something. We don't have knowledge of people, we have the experience of being in contact with people, so that "conosco questa persona". There are cases in which both can be used but they would not mean the same thing. For example, "Conosco la legge di gravità" means that you have already heard about it, whereas "so la legge di gravità" means that you know what it says and could explain it. In addition, this is the same case in Spanish and Portuguese.
In this example, la is a direct object pronoun. In English, it is placed after the verb. In Italian, it is put before the verb.
mi.....me; ti.....you; lo.....him, it; la......her, it; La (fml)........her, it; ci......us; vi (pl.)......you; li (m.)......them; le (f.).......them;
EX: Vedi Anna stasera? No, la vedo domani. Conosci Paolo e Maria? Si', li conosco.