From CAP804:
An applicant who has held an ICAO IR(A) or military green aeroplane Instrument Rating or a UK professional pilots licence aeroplanes within the 10 years before the date of application for the IMC Rating is not required to undergo a course of flight or ground training but must pass the Theoretical Knowledge examination and initial IMC Rating /IR (Restricted) Skill Test.
BUT:
An applicant who has held an ICAO IR(A) or military green aeroplane Instrument Rating or a UK professional pilot licence aeroplanes or a valid Aircraft Owners and Pilots Association (AOPA) Ground Instructors Certificate within the 5 years before the date of application for the IMC Rating / IR (Restricted) will be credited with a pass in the Theoretical Knowledge Examination.
So it would seem that he would just need to pass the IR(R) Skill Test, then apply to CAA to have the Rating included in his licence.