"He used to know her."
Translation:Él la conocía.
Saber is to know factual information, like telephone numbers, or calculus, or know how (with infinitive); conocer is to be familiar or acquainted with a person, place, or thing
"Conocer a" in the present means "to meet." Maybe your sentence means "he was meeting her?"
because if you are going to use a pronoun "her" in a sentence in Castellano, there is already a sentence structure for doing so. "él la conocía". If you wanted to add "a ella" to clarify what "la" is, fine, but you can't drop the pronoun in doing so - él la conocía a ella". Hope that helps