In the original post you mentioned that there were several other aircraft on frequency. It is probably worth a thought to how you might react on hearing a mayday transmitted on the frequency in use.
The distress aircraft only needs to know that someone has heard him, they can then get on with the business in hand.
If you hear a mayday in flight that is not acknowledged by ATC then you should reply with 'C/S this is C/S roger mayday out'. Then on either the freq in use or 121.5 you should broadcast 'mayday relay X3 this is C/S'. When asked to pass the message, pass it exactly as you heard it and be sure to emphasise the callsign that you are relaying for so that you are not mistaken as the aircraft in distress.