Years ago I moved flats and had taken all my fish and the water out of my tank and only left the stones in it (it was a small tank by my standards).
Anyway when I refilled it hours later all these platty babies came from no where out of the stones. Didn't even know I had them. Also had black neon babies survive like this once.
Guppies and plattys etc are pretty tough little fish.
Did he have any fish where the oxygen weed came from?