See if anyone can bring a laptop to the field with a flight sim on it. Since you are recovering the plane, there appear to be no negative consequence to mistakes made (and hence no learning).
i normally let the mistake go as far as i can to give the student every chance of recovering them selves, normally watching it get that low and scary is good enough, however if they can now fly reasonable and are just mucking about rather than doing what i tell them ( plenty of time to muck about after you go solo, and when i give them a few mins to settle in at the start of the flight ) i take it back, this only works with some people....
Second, when I was (the learner) on a buddy box, I had no real way of knowing when the instructor had taken over. Mabybe that's the problem here.
i do make sure i say, ok i got it, as i take over, i then take it back up, do a circuit so they relax again and then give it back.
however that does lead to the problem that they ask if i did take it even tho i didn't say anything ( coz i didn't take it ) and they are sometimes surprised to find out they did it, a good ego boost for them, but worrying too for me... they should know really...