Not required
There is no requirement in Europe for the 'student' pilot in the left seat to have a valid medical if on a dual flight with an instructor who is the PIC and has a valid medical.
Of course if the student was to go solo then a valid medical is required, goes without saying and so virtually every student will have a medical and this issue never arises.
If there is any training done which does not require solo flights such as lets say CPL training or IR training, again a valid medical is not required whilst you have an instructor on board, indeed you can even do your skills test without a valid medical as your examiner is the PIC and will have a valid medical, however you won't be issued your CPL or IR until you have a valid medical.
This chap may have his own rules for his FTO within his Ops manual but it is not a legal thing issued by EASA.