Every person who shall have been concerned in any gaming contrary to this Act, and who shall be examined as a witness by or before a Magistrate on the trial of any charge against the owner, keeper or occupier or other person under sub-section (1) of section 78 or under section 79 touching gaming, and who upon such examination shall make true and faithful discovery to the best of his knowledge of all things as to which he shall be so examined, and who shall thereupon receive from the said Magistrate a certificate in writing to that effect, shall be freed from all prosecutions under this Act for anything done before that time in respect of such gaming.