The problem is, if she is his GF, and he is planning to live with her for rest of their lives, it is kind of a problem, when one (he/she does not matter) do not follow the sane money management (or simply MMM)..
Because one of the two drags down the capital of the family.
And I believe it does not matter, if the finances are shared or not.
Because she will, in this particular case, she will blow away the money on car, instead of, for example a holiday (her part/half).
My GF is the same, she likes some idea, crazy or not, it's always expensive. And as folks above said, communication needs to be changed, you need to explain her differently, best time for it is when her mood is good or so...
Try to explain her, or send her link for this topic:
https://forum.mrmoneymustache.com/share-your-badassity/i-just-bought-2-days-per-year-of-free-life-indefinitely-363-to-go/For 2K she would waste, she could have 2 days of retirement every single year for ever.
Also, it is VERY questionable, which car is in better or worst state - no one is going to sell a car in perfect shape. They would sell it to their family, maybe, I guess.
For example, my car is made in JP, has 138k miles on it, in just 5 years. I could sell for, according to some online listings, 10k maybe.. but what would I buy? new for 10k is not a good idea, unless Dacia.. so I would risk more by selling it, and replacing it, because today I know, I did behave to the car nicely and it will work well.. if not, I was wrong.
The worst part of this is, at least in my opinion, that we want our other halves or family to be FI, but they are making moves (deliberately or not, conscientiously or not) which delays, if not totally ruin the FI plan. Well, marketing and brainwashing is a strong tool/virus.
i did buy so damn useless things and so expensive, like sofa for 5K, i will regret it. Also, I did change my 2L gas engine for larger with turbo, I did lost 10K and maintenance is more expensive. With sofa, its instantly 15K down, and perhaps even more (higher insurance and so on), so I am down 20K with just 2 mistakes. If i would invest it in stocks (even safe one, dollar cost average and so on), I would be on 30K for sure. And that is 1 MONTH of FI every year, approx. let it do it's job , invest it, and I would be on 60K within 8-10 years. damn my mistakes..