Had a similar situation about a week ago. Driving to work in the dark along unlit rural road, came up behind a colleague also driving to work who had turned onto the road a couple of hundred yards in front of me.
Before my colleague had pulled onto the road I had seen a bike some distance further up the road and when I passed the cyclist I muttered to myself that he could improve his own self-preservation by not riding in head to toe dark clothing with only a single, dim rear light.
When I got to work my colleague was having a right rant about the idiot cyclist he’d only seen at the last moment and had nearly hit; it would have been his own stupid fault for riding in black clothes with no lights.
I pointed out that I had been following behind him and had seen the bike light from some distance. His reply was “yeah, but you’re a bike rider”.
The outcome was that we agreed that the cyclist could have been better lit and wearing more visible clothing but also that my colleague should have seen him in any case and he conceded that he should be more observant driving rural roads in the dark.
The one difference to the OP is that my rider was actually riding within the law; he did have (minimal) lights front and back.