Earlier quoted context omitted.
The USA charges people it evacuates from war zones, and may or may not do so at all. It is not obligated to do so. Additionally, if she has dual citizenship, she would get it for free from the UK anyway even if she lives in Spain. People with British passports also have 90-day visa free access under the Visa waiver program. Unless she intends to move there, there aren't really any benefits to being a US citizen, and…
If she's come to the conclusion that she doesn't need any of the services that she gets as a citizen, for the reasons you've given, then why is she still a citizen? Either it's worth it (including any emotional attachment) when you balance it against the cost or it isn't. But if she chooses to retain her citizenship then she's incurring a potential cost to the US, which surely it's reasonable for her to contribute to…
Why is she still a citizen? The article says she's in the process of renouncing. Not done already probably because:
• She might one day need to spend time with family or elderly relatives, even if she'd prefer to stay abroad.
• The USA does not allow you to give up your citizenship for tax reasons. She's probably screwed herself here by talking to the BBC, so I suspect she doesn't know that (edit: wrong, I missed the part in the article where it says Jane is a pseudonym). If they think you relinquished your citizenship to avoid taxes they can simply levy taxes on you anyway.
• Giving up citizenship is itself an expensive process and can trigger an "exit tax" that assumes you liquidated every asset you own on the day of renouncement, including things like a home. This exit tax can itself make giving up US citizenship financially infeasible.
• It costs $2350 and can take months.
In short, the entire US FATCA system is designed to screw over expats as hard as possible. It's very hard to get out and isn't at all justifiable under any moral or ethical code I'm aware of. The only reason the US can enforce it at all is the primacy of New York in the financial system and the incredibly aggressive Congress.