I feel for the Remainers, especially those living or working internationally. The result of this lawsuit isn't very surprising at all (turns out leaving the EU means you don't get EU benefits anymore and your country's limitations on voting aren't another country's fault) but sadly what's done is done. The UK didn't want freedom of movement/settlement with the EU and it was clear from the start that the EU wasn't going to grant rights to UK citizens the UK wasn't going to grant to the EU.

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.