I can see both sides of the argument and make no judgement because it is impossible to know what the second pilot could actually see.
Firstly if, as alleged, this aircraft was in solid cloud, for a mid-air collision to take place (this is what people are concerned about), it would have required another aircraft to be in the same place, same height, same time, under an ATC service.
Secondly, it is sometimes not possible for a pilot to see another aircraft, even in good VMC. What should he then do - declare IFR?
A CHIRP report would possibly achieve little except appease the contributor; the accused pilot may not be a member of the scheme. If the ATCO was concerned that rules were broken, he could submit an MOR. Unfortunately, for a pilot to "report" another for an alleged demeanour has as much chance of "success" as reporting another motorist for speeding or holding a mobile phone.