A lone witness is not sufficient to establish any wrongdoing or sin against a man, regardless of what offense he may have committed. A matter must be established by the testimony of two or three witnesses.
Jewish Publication Society of America Version (JPS1917) - Deuteronomy 19:15 >One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth; at the mouth of two witnesses, or at the mouth of three witnesses, shall a matter be establishment