I looked into this a little while ago - as an ex-teacher with files of resources that I happily shared with my colleagues of the time.
It is true to say that the resources created while you are employed are considered to be the intellectual property of the school/employer.
www.copyrightsandwrongs.nen.gov.uk/schools-a-copyright/teacher-the-workforce-the-school-and-copyright
However, I think that this would actually be very difficult to enforce as the school would have to demonstrate that they have suffered a loss in order to be awarded any sum in recompense - Mrs Smith, a highly valued teacher from Reading makes £100 a year selling resources. Are the school really going to take her to court for that?
When I looked into this it also quickly became apparent that the resources published for sale on TES have some fairly significant changes made prior to publication, over and above what you might use in the classroom. For example, all images need to be copyright free, the resources tend to be produced with lots of graphics/DTP features and they tend to be 'bundled' along particular themes - so really not the same as the sheets or resources you might have produced under time pressure in your PPA time. So if you have thought a up a nice way of teaching a particular topic, used it successfully in the classroom, then put in additional time and energy into making it ready for publication - then is there really anything wrong with being rewarded for that? The requirements were certainly enough additional work to make me shelve the idea for now, once I realised that I couldn't just upload a few sheets and watch the pounds pennies roll in!
Remember too that there are hordes of freelance trainers, advisers and consultants going around the country selling INSET sessions to schools on the basis of the training, experience and development they received when they were employed by schools and LAs. Or should that be cracked down upon too?
It might be too late, but I would tell your mentee that you really value her resources and if she wants to also sell them online, then although the IP really belongs to the school, you are happy to turn a blind eye.