Some people really do not realise how badly they can hum in the cockpit! There is a point where you **HAVE** to intervene - and having been there done that myself, I can't say it was fun for me nor the malodourous student!
Surely by briefing on this before getting underway with the course, people who shower regularly can bring their sloping shoulders into action or likewise those that don't can avoid an embarrassing situation!!
I wouldn't take offence to such a thing.