Use of another's key is prohibited

No, this would be a violation of the rules and regulations and is a finable offence. REIN Rules and Regulations state:

Not to disclose or to make readily available to any third party the Keyholder's PIN. Not to allow the Keyholder's PIN to be displayed on or attached to the Key. Not to loan the Key to any person or entity for any purpose whatsoever or to permit the Key to be used by any other person or entity whatsoever, whether or not such other person is a real estate broker or associate.