Trial by pprune again is it? (I've skipped a few posts but I can get the gist).
Nobody but the pilot knows their conditions
Both were under a radar service
No safety was compromised
So what are you going to report? That you didn't "think" the other pilot was in VMC?
I once asked for IFR in the USA (entering LA basin from Banning Pass) coming back to Long Beach and the vis was getting bad, and I feared it may get worse. I was told to hold and that "time now is 20, expect further clearance at 44". It was still VMC, all be it at minimums and a sun going down, so I cancelled straight away and continued VFR with a radar service. Coming into Long Beach there was this bloke on the radio being told to descend to 1500' (our altitude) and he was winging and complaining that he couldn't maintain VFR at 1500'. So the controller asked us our conditions and we verified that it was indeed legally VFR. I say legally VFR because according to the definition of VMC we could still maintain VFR. I don't like marginal conditions which is why I asked IFR in the first place, but I also didn't want to spend 24 minutes in the hold. (we got a practice ILS to make sure we didn't miss the airport
).
My point is that this one bloke above us was complaining that we could not have been VFR when in actual fact we were in VMC....get the point?