What Stitcher said. The Civil Partnership and Certain Rights and Obligations of Cohabitants Act 2010 Pt 15 is mainly about protecting cohabitants in the event of relationship breakown, which of course never happened in Louis's case. It deals give the courts power to award maintenance to qualifying cohabitants, make property adjustment orders in their favour, etc. It does also cover cases of death; if a surviving qualifying cohabitant reckons they have been left high and dry by being omitted from their deceased partner's will or getting only a trivial amount under it, they can apply to court for an order that proper provision be made for them out of the estate. .
Pt 15 doesn't deal with tax at all, but it's supplemented by Capital Acquisitions Consolidation Act 2003 s. 88A, which says that a gift or inheritance to a qualified cohabitant that is ordered by the court under Pt 15 is exempt from CAT.
It wouldn't be enough for Louis to get an order from the court saying that they are a qualified cohabitant of the deceased. They would also have to get an order giving them a part of the deceased's estate. The court can only make that order if it is satisfied that the deceased failed to make proper provision for Louis, which is a problem, since the deceased made Louis one of the primary beneficiaries of their estate. Plus, the max that the court could order for Louis is what a spouse would get as a legal right under the Succession Act, which may be less than what the will gives Louis. So I think Louis is snookered here; I don't see a realistic pathway to a Pt 15 order, which means CATCA s. 88A won't apply to the case.
And there's no parallel provision in CATCA saying that a gift or inheritance freely given to a qualified cohabitant is exempt from CAT. The thinking there, evidently, is that if the couple wanted a CAT exemption they shoulda put a ring on it. (See also: joint assessment for income tax purposes.)
A couple of other "it's too late to say that now" points:
1. If you're in a situation where you're looking at the possiblity of a big CAT hit in these circumstances and, for whatever reason, you can't or don't want to avoid it by marrying, you can get insurance for that. Not free but, depending on the circumstances, can be a lot less than the CAT hit would be. It can be useful, e.g., if you intend to marry but not for a couple of years, since the risk of one of you dying in the next couple of years is usually quite low, and the cost of insuring agains that risk will be commensurately low.
2. The solicitor who drew up these wills for Louis and their fiancé should have pointed out this issue. The couple could then have taken a decision about whether to accelerate their wedding plans or take out insurance or whatever. If Louis and their fiancé took professional estate planning advice and this issue wasn't raised with them — well, I hesitate to encourage anyone to embark on a prefessional negligence claim (it will take years, and suck all the joy and energy out of your life) but that option is there.