Randomly wandering over youtube links I was surprised to hear a recording of a BA crew informing ground of a fire on another a/c using "mayday, mayday, mayday".
https://www.youtube.com/watch?v=jIt6CDLrPOo (4'30")
In my world (commercial shipping) I would never call mayday on behalf of another vessel, because of the potential for confusion. We strictly use the phrase 'mayday relay' for that purpose. That is obvious if you are 'relaying' a mayday radio message on behalf of another vessel, but it is equally applicable if you see a visual distress signal or another vessel in distress.
Just wondering if you chaps use the same conventions?