"Do not listen to her, she does not know what she says."

Translation: Nie słuchajcie jej, ona nie wie co mówi.

[AndrejCzapszys]

Is there a rule for when to use jej versus niej?

[mihxal]

Yes, "niej" is used with prepositions, "jej" is used in other cases when you have to use Genitive.

[SonofMaat]

Why not 'słuchaj' please?

[Jellei]

It works.

[dns.dns]

is jej the only option or can I also use nie słuchaj ją? thanks.

[alik1989]

It's the only option, because the "negated accusative = genitive" -rule always applies.

[VeryBigP]

Is "co mówi" not the direct object of "nie wie"?

idk, I tried "ona nie wie czego mówi"