If a Mayday has been issued by someone.. all non mayday traffic is supposed to remain silent until told by station issuing the maday that radio silence can be finished.
From the Transport Canada AIM:
Com 5.11:
The distress call shall have absolute priority over all other
transmissions. All stations hearing it shall immediately cease
any transmission which may interfere with it and shall listen
on the frequency used for the distress call.
From the Study Guide for the Radiotelephone Operator's Restricted Certificate (Aeronautical) RIC-21:
Page 19:
Should radio silence be imposed during a distress situation, all transmissions shall
cease immediately except from those stations involved in distress traffic.
Examples: Imposition of silence on a specific station by the station in distress.
(Cessna C-FNJI is causing interference to distress traffic.)
CESSNA FOXTROT NOVEMBER JULIETT INDIA
THIS IS
PIPER FOXTROT X-RAY QUEBEC QUEBEC
STOP TRANSMITTING - MAYDAY
OUT
Imposition of silence on all stations by a station other than the station
in distress.
ALL STATIONS, ALL STATIONS, ALL STATIONS
THIS IS
CESSNA FOXTROT NOVEMBER JULIETT INDIA
STOP TRANSMITTING DISTRESS
OUT