any damage was likely already done in an enclosed cabin recirculating air...depending on where he was sitting relative to recirculating air intakes.

Yes, people sitting next to him would be slightly more at risk, as they would have been anyway - mask or no mask - sitting beside someone for ~8+ hours.

IMHO, should have just gone on to London - then cancel his return ticket as a fine - and release his name to other airlines.

This isn't anti-mask - for most things I'm a when in Rome do as the Romans do type guy, I wear one where "required" without complaint, but also think we should use some common sense for solutions.