MAYDAY RELAY is an acceptable term for use by sea craft, but has not found its way in to aviation RT phraseology.
In UK airspace, CAP 413 offers the following advice:
1.9
Relayed Emergency Message
Any aeronautical station or aircraft knowing of an emergency incident may transmit a distress message whenever such action is necessary to obtain assistance for the aircraft or vessel in distress. In such circumstances, it should be made clear that the aircraft transmitting is not itself in distress.
Example:
Aircraft G-ABCD
MAYDAY MAYDAY MAYDAY
Milthorpe Tower G-ABCD have intercepted MAYDAY from G-BJRD
I say again G-BJRD Cessna 172 engine failure forced landing 10
miles west of Wicken VOR, 1000 feet descending, heading 120, IMC
rating, over
Milthorpe Tower
G-ABCD Milthorpe Tower Roger your relayed MAYDAY from G-BJRD
You can only be relaying a Mayday if another station has transmitted one.
But that's not what CAP 413 says. Read it again. above.
For a sighted liferaft, you could use a MAYDAY call quite legitimately, provided you made it clear you were under no emergency on board your own aircraft. Alternatively, you might assess it as only worthy of a PAN, but that's your call. The emergency services would be more appreciative of a call and it being a false alarm rather than finding out later someone was in trouble and you thought they were only using a liferaft for fishing !!