The reappearance of the issue of the world wide real estate owned by the Jarvisses has made me think of two other hazinesses in her story lately: her main bank account being British and her sudden need for a carte de sejour, when she should have had her French passport by now.
The French tax system taxes real estate wealth through IFI, which has replaced the solidarity wealth tax (ISF) since January 2018. One is liable for this tax if the net value of your property worldwide exceeds 1,300,000 euros. With just her London flat and the HMN she would definitely be liable for it, but: if you do not live in France, only real estate located in France is taken into account, which is probably also why they are trying to keep HMNās actual value as low as possible, not increased over the years since they bought the place despite the work done to it and not even taken inflation into account.
Claiming to be a British citizen, owning a holiday home in France (HMN) and spend time there through a carte de sejour instead of a passport (which would imply citizenship) and using a British bank account she is clearly avoiding this tax, which rate varies between 0.50% and 1.50% of the declared value every year. Her multiple trips also play into this narrative of only using the HMN as a holiday home.
It is up to you to decide whether or not to declare your assets to the IFI. However, and SJ should pay close attention: if the tax authorities decide that you are subject to this wealth tax nevertheless, they have the right to collect the arrears with penalties over the previous TEN years. Maybe she should inform her patrons of the risks she is taking with their money?