I am an Instructor and so is backpacker.
These runway changes with student circuit traffic have always led to bum clinchers and near misses. ATC don't seem to realise how little capacity the solo students have. Unfortaunately the only way of them increasing it is by flying solo.
I had a similar experence at 5 hours with an instructor onboard during my PPL. He never spotted it until I climbed away. Again this was on a runway change because of some tosser coming in a Biz jet straight in to the oppersite end to the current circuit direction at a UNicom field.
Yes the OP is a naughty pilot for turning in without seeing the number one traffic. You can be assured they won't do it again. I never think there is much point labouring the point with debriefs after a student has !!!! themselves. Just confirm that they know what happened, then confirm that they know how not to let it happen again. If they haven't a clue its back to dual flying for a while until they do.