UK AIP ENR 1-5-4 defines established as half scale for ILS, VOR and 5 degrees for NDB. Outside these limits means you are venturing outside the protected area, and terrain separation reduces, eventually to zero.
You need to look at how an approach is constructed in order to show this. If they have studied ATPL exams, then refer them to their course notes. I would generally say (if you are teaching IR) that the discussion is beyond the scope of the flying element of the course, as it was covered during ATPL's.
Pans-Ops is the place to go if you really want to get something in black and white, or your company ops manual should cover it if you have one.
Hope this helps.