If you just pick an emergency at random in flight, there are only two possible outcomes, one is that it all goes according to plan, in which case it was a waste of flight time, or it doesn't go to plan in which case the PPL merely learns that he can't do something.
No, if it goes to plan then you have confirmed the student knows the emergency and how to handle it - that is a big plus as far as I am concerned and you can praise him for it. If he messes it up (providing it is one he has been shown/practised before) then you make a learning point from it, highlight the indications/actions/remedy and then he has learned something - he has learned/relearned that one which he might not have fully understood before.
In each case the student learns - that is surely the point of the job of an instructor.
The only negative way of handling it is if you berate him for messing up the emergency and then make a big show of how clever you are at demonstrating the right way of doing it - that is negative teaching but it amazes me how many instructors don't seem to be able to tell the difference between instructing and destructing.