I know of a man that dresses as a priest (wears clerical blacks and has the Roman collar). He even uses this disguise for his legal photo identification. He has even changed his name legally to reflect this false personality. He doesn’t live the life of Christ (and I won’t give examples of his behavior). Can I do more than pray? Would having a Mass offered for him help?
From your account, it is impossible to discern what you mean by a “false priest.” Are you referring to a non-Catholic Christian minister who happens to wear clericals as part of his ministry, or to someone actually trying to convince people that he is a Roman Catholic priest?
If the former, please be aware that some non-Catholic Christian ministers do wear clerical blacks, the white collar, and are referred to by the courtesy title “Father.” While the Catholic Church does not recognize the validity of holy orders in ecclesial communities that have lost valid apostolic succession, it would be uncharitable to refer to a non-Catholic Christian minister as a “false priest.”
However, perhaps you intend to say that this person is actively trying to convince people that he is a Roman Catholic priest. In that case, call the Roman Catholic diocese in which he lives and inform the staff of the situation. That way, if the diocesan staff is asked about this person’s priestly credentials, their office will have knowledge that the situation exists and information prepared to give inquirers.