My friend is deaf person and I help him to fill out some paper. He wants to apply for USA Passport. He is citizen of USA, but he lost his citizenship certificate. How he can get duplicate of citizenship certificate?

I think this might be what you're looking for:
http://www.uscis.gov/portal/site/uscis

Application for Replacement Naturalization/Citizenship Document
Purpose of Form :
This form is used to apply for a replacement Declaration of Intention, Naturalization Certificate, Certificate of Citizenship, or Repatriation Certificate, or to apply for a special certificate of naturalization as a U.S. citizen to be recognized by a foreign country.
Number of Pages : 4
Edition Date : 10/26/05. Prior versions acceptable.
Where to File : File this application with the USCIS local Office with jurisdiction over your place of residence.
Filing Fee : $220.00
Special Instructions : Note on Filing Fee:
Please consult the "Paying Fees" section of your local office page to learn which forms of payment are accepted. Acceptable forms of payment may vary by office. Checks should be made payable to Department of Homeland Security or U.S. Citizenship and Immigration Services.

Download N-565 (797KB PDF)