There is no requirment what so ever that the DI is working for doing PPL lessons.
I don't doubt that but would suggest that charging the current rates for flying such a heap is totally ripping off the customer, who is most unlikely to fly in such a piece of junk for real afterwards.
I think a perfect DI will indeed drift 15 deg/hr (doesn't it depend on where on the earth's surface you are?) but in my PPL training and subsequent self fly hire, all done with non-slaved DIs, I never saw one which was better than about 10 degrees every 10 minutes. That means constant resetting from the compass is needed, which is OK in still air
