The N-336 should only be filed if you can overcome the reasons for the denial at the N-400 interview. Given that your mother was unable to pass the English test after two attempts and having lived in the US for a minimum of three years, it is unlikely that she would be able to overcome the reasons for the denial, particularly if she is constantly traveling to her country of origin.
How can she prepare? By studying English.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
Why are you appealing the denial? That is what the N-336 is. She should brush up on her English if she does not qualify for the waiver of English, and then apply for Naturalization again.
An attorney-client relationship is not formed by my responses to questions on Avvo. My responses are not intended to be legal advice and must not be relied upon as legal advice.