YOU SAID:

He said to her that she shouldn't have said that to him about their kitten

INTO JAPANESE

彼は彼女に彼らの子猫について彼にそれを言うべきではなかったと言った

BACK INTO ENGLISH

He told her she shouldn't have told him about their kittens

INTO JAPANESE

彼は彼女に彼らの子猫について彼に話すべきではなかったと彼女に言った

BACK INTO ENGLISH

He told her she shouldn't have told him about their kittens