As for the meaning, it’s “What has happened to her?” or rather “What has become of her?”. This idiom implies that you have not heard of her in a while, haven’t had the chance to get any information about her, and don’t know what her fate is.
It might be needless to point out that the meaning of ”ne” in this idiom is useless to understand the meaning of the idiom as a whole.
Anyway, with this warning in mind, you can construe that ”ne” as a sort of demonstrative pronoun (=‘da ciò’, “from that”). As you can see, the meaning of the pronoun is blurred.
That is confirmed by the Vocabolario Treccani, s.v. “Essere”, meaning 3e, where essere combines with a particle with no clear-cut meaning:
Avvenire, accadere […] con la particella ci o ne: che c’è?, che
cosa succede?; chissà che ne è stato di lui.
Moreover, Luca Serianni in Italiano, Milano: “Garzanti”, 2000, § VII.55 wrote:
In molte locuzioni cristallizzate il valore di ne, locativo o pronominale, appare attenuato, quasi irriconoscibile.