I think @Strima has already answered this question perfectly.Officially there's no requirement to hold any qualification to carry out periodic inspections but you must be competent. But to prove competency the easiest way is to hold one of the following qualifications, others are available:
C&G 2391 level 3 the older qualification
C&G 2391-51 Level 3 Award in Periodic Inspection and Testing of Electrical Installations.
C&G 2391-52 Level 3 Award in Initial and Periodic Inspection and Testing of Electrical Installations.
C&G 2392 Level 2 Fundamental Inspection and Testing qualification.
EAL Level 3 Award in Electrical Installation Inspection, Testing, Certification and Reporting.
Other qualifications are available.
C&G 2382-18 is only an open book exam to show you know how to read and extract information from BS7671 and in no way covers you for periodic inspections.
also, although not a requirement it would be a good idea for him to join one of the competent person schemes such as NICEIC or NAPIT.
this is a great way of proving competence because they check your qualifications, insurance, and through inspecting your work keep an eye on your knowledge.