You mean he has a frozen ATPL without IR?
He has ATPL groundschool, and a CPL. I hate the phrase frozen ATPL, since noone seems to know what they actually mean by it.
If he lets the exams go, he wouldn't have to do a full registered course again, he would have to do "training as required" and then retake all the exams at a cost of £1000+.
If you go ahead and get your IR, a full renewal could be £700, or even less if you know where to go, so it would make more sense to just get the IR now if you have any plan to use it in the future.
If you are more confident that you will not need the IR at all, let the ground school go and be prepared to take the hit in the future. Perhaps better to take the risk of having to spend an extra £1000 or so in the future, rather than spend £15,000+ now and then realise it was a waste.