I know a person who is about to have his interview for citizenship this coming 18th of August 2016, he is a hard user of illegal drugs before,he wrote a false statement in his application because there is a question there asking if you have used illegal drugs before, of course his answer is no, where can I report this kind of fraud?

You can report anything you want to Immigration, and you have plenty of time, if this is the beginning of his process. But can you prove it? Countless people report anybody and everybody for reasons of spite or suspicion or they are angry about something. (This is why the FBI has such a tough time checking thousands of suspected terrorists.)

Your sentence is a bit confusing too, because you say he IS a hard user BEFORE. Which is it? When was the last time you know about, personally? Was he ever arrested, charged, convicted?

Go online to INS and look for reporting info.

Unless he was arrested, or you have some legal proof, you are wastng your time.

Undocumented claims (no proof) would be considered but really would not stop anything. He could merely claim you have a personal problem or issue with them