Had a similar request myself about 2 years ago from a distant (very distant) relation who was from South Africa but with no rights (parental) to UK citizenship. Answer was, regrettably, that he would require to achieve UK citizenship by fulfilling the residency (3 years IIRC) requirement. Commonwealth doesn't cut it these days unless HM Queen is your head of state; that does not apply to RSA.
Best thing he can do is approach the recruiters in the High Street for the definitive 2005 update!