This could very well affect his application. One requirement for naturalization is showing good moral character, and USCIS will look into the last five years in particular. You may need to wait until five years have passed since the assault. However, depending on the circumstances of the assault, it might be something that could affect him even after 5 years have passed. Before applying, he should consult with an immigration attorney first who can review all the facts involved.
The information offered is general in nature and not meant to be relied upon as legal advice. No client-attorney relationship is created through this information. Please consult an attorney prior to making legal decisions.
It can impact his application to naturalize. USCIS will conduct a background check and determine his good moral character. If this conviction is a legal preclusion (see INA 101(f)) it would bar the approval of the application. If it is not a per se preclusion, they can still use it to determine he lacks good moral character using the catch all clause in the above noted statute. I highly encourage you to retain experienced immigration counsel.
I agree with my colleagues ... he needs to consult with an immigration attorney before filing the N-400.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.