Thanks for your replies folks - reinforcing my own view I think. The instructor is apparently a 747 captain or FO, so experience should not be a question (though I don't know what his experience of flying relatively low in small aircraft is).
P. Pilcher - MSA was above freezing level so descent was not an attractive option - this is what made it particularly bad for me. They apparently flew an airways route almost permanently in cloud. It sounds like they were picking up rime - sufficient to alarm my friend when he heard chunks of ice hitting the fuselage from the props...
I've pointed out the CRM aspects of this to him too - he was concerned about the build-up but the instructor said it wasn't a problem so they carried on. At the very least, it's teaching an inexperienced IR student potentially bad/ fatal habits. Quite aside from flying other than in accordance with the AFM.
I've advised my friend to ask that instructor (tactfully) more about his decision and view of icing - it may reveal something that he can either learn from or use as his own reason for not flying with that guy again.