YOU SAID:
She knew he cheated on her
INTO JAPANESE
彼女は彼が彼女をだましていることを知っていた
BACK INTO ENGLISH
She knew he was fooling her
INTO JAPANESE
彼女は彼が彼女をだましていることを知っていた
BACK INTO ENGLISH
She knew he was fooling her
That didn't even make that much sense in English.