I do hope the tensions around the northern Irish border situation won't impede UK citizens even more in the years to come. The way the UK seems to be coming back on their promises paints a grim image for things to come.
Headlines today say that the UK government will come out with a proposal next week that would violate the current border agreements. The current customs situation is an enormous hassle for the Northern Irish people, basically forming a customs border within the Kingdom, and the UK government want to cut through the red tape. That's not necessarily bad, it makes complete sense, but it's made difficult by the fact the easy solution (just putting a border between the two Irelands) isn't feasible.
If the UK decides to unilaterally alter the deal, Ireland may be required to enforce their customs policy along the border, which is feared to spark more violence in the area. After all, Ireland is either in the customs union or it's not, it can't become a free corridor for UK goods to enter the EU.
Sinn Féin and the DUP being at odds with each other after the latter lost the election certainly isn't helping. This whole situation is a mess and that's why I think the UK government altering their deal will only cause problems.
I hope that cooler heads prevail here. The rumoured proposal is still just that, a proposal. If it gets shot down, or if it gets held back until the situation has cooled down a bit, this might just turn into yet another round of EU negotiations.
A majority of people in NI recently voted for parties that support the NIP, and NI and London are the only regions in the UK that are currently experiencing growth. I think it might be more opportunity than hassle.
The only recourse here would be for UK citizens to apply for European citizenship in a member state of their choice, but that's a long, complicated process, not dissimilar to the process of becoming a recognised UK citizen. It's also incredibly difficult if you're not highly-skilled, already living abroad and/or have access to exchange programs that offer a way in, such as access to education.
For the person in question, this should be easy: France requires that you've lived there for five years, have knowledge of France and speak French, something I'd expect should be quite doable if you've lived there for fifteen years. The UK allows for double citizenship, so I think there's an easy way out for people in such situations without giving up any rights in the home country, much easier than for most people who come to France from outside the EU.