YOU SAID:

she can’t tell her what she doesn’t know because if she did that would mean that she knows when actually she doesn’t.

INTO JAPANESE

彼女が知らないことを彼女に伝えることはできません

BACK INTO ENGLISH

I can't tell her what she doesn't know

INTO JAPANESE

私は彼女が知らないことを彼女に伝えることができません

BACK INTO ENGLISH

I can't tell her what she doesn't know