Without being a "fly on the wall" it is very difficult to give any form of definitive answer to this. It is however a fact that instructors and newly qualified CPL's have had little or no icing experience even in aircraft that are certified for flight in such conditions thus it is very difficult to criticise this particular instructor as we do not know how much experience he has had. In his favour the freezing level was above the MSA thus if things did turn really pear shaped, he could have descended (or may have been obliged to) and normal services would have been restored before an encounter with the hard stuff would otherwise have occurred. How light was the dusting? Had he penetrated briefly a wisp of cloud below 0 degrees C and picked up a little dusting, or was he holding or conducting an airways sector in the stuff which was building up continuously. If either of these was the case then he had clearly assessed that the ice build up was sufficiently slow that he would not experience trouble and would in any case melt it off when he descended. As more experienced bretherin will know, even with a fully de-iced aircraft, it is best to keep out of it if at all possible. Expanding boots on leading edges are not always successful and ice invariably builds up elsewhere on the airframe increasing weight, impairing performance and slowing the a/c down. I don't know how many times, in this situation I have said "10 more knots off the airspeed and I am descending to get rid of this!"
No - I would not castigate this instructor as some have, but if he was picking up clear ice from supercooled rain and the 0 degree isotherm was below his MSA, then I would heartily agree.
P.P.