If he completes the CPL theoretical examinations and only wishes to obtain a PPL with FI certificate, there is no expiry of those CPL theoretical examinations for this purpose.
Which is why there is little point being persuaded to taking the ATPL exams if you only wish to instruct!
to pick a 'student' up from one place and give them a 'trial lesson' culminating at a landing at another place, being paid in the mean time...
As all training has to be conducted at an Approved School there is no scope for independent activity.