YOU SAID:

I don't know if they know that she knows about him

INTO JAPANESE

彼女が彼のことを知っていることを彼らが知っているかどうかはわかりません

BACK INTO ENGLISH

I don't know if they know she knows him

INTO JAPANESE

彼女が彼を知っていることを彼らが知っているかどうかはわかりません

BACK INTO ENGLISH

I don't know if they know she knows him