All the EASA rules say is that all the theory exams have to be done under the jurisdiction of one single State so yes, once you have jumped through the hoop in one State (*jumped through the hoop* meaning *passed all the exams*) you wouldn't have to do it again in another. The UK CAA are stretching this to allow PTC students to finish off in the UK but no-one is going to let your chap off the requirement to have his exam application signed off. I would suggest:
1. If he has completed all his study in Ireland and has been signed off for the exams as you suggest in your final para then he doesn't need to do anything other than take the exams and there he should probably do them there.
2. If he has completed his study but can't get a sign-off in Ireland he will need a sign-off from a UK FTO. This will require his training records to be transferred and maybe some form of crammer. (Watch out, if he was doing an integrated course he can't transfer to a modular FTO unless he has a PPL)
3. If he hasn't started or only just started the study for the four remaining exams he should probably find a UK or Irish FTO that can do that element of the approved course, this may involve new course materials etc. (Caveat as above).
The interesting implication of this slight change in the rules is that it is more likely that people will shop around in Europe for CPL courses. Shopping around for IR courses already happens.