If you are referring to Basic Allowance for Housing (BAH) fraud, then I assume you are affiliated with one of the branches of the U.S. Armed Forces. If you suspect that someone is claiming entitlements for a housing allowance that they are not entitled to receive, you can report your concerns to the Base Housing Office or the military Inspector General (each branch has a dedicated IG.) Be prepared to provide facts and support for your allegations. If you are worried about possible reprisals, most military bases have a confidential hotline that allows you to report suspected fraud anonymously. Good luck and have courage to do the right thing.
This is a general comment to your posting and is not meant to constitute legal opinion or advice. There is no attorney-client relationship in this matter. If you think you need legal advice or representation for this matter, you should consult a licensed attorney in your jurisdiction.