At her age, I wouldn’t want to live with a host family. Too much risk of incompatibility. It may make it hard to socialise with other young people - you can’t just ask them home.
It’s true that London didn’t lately seem very English anymore, it was majority immigrant when I was there a few years ago. I’d still call it “British” though. A true melting pot and international city, more so than NYC. This may have actually changed somewhat with Brexit and Covid, Britain has lost quite a bit of its foreign born population and most will have exited London.
In her case, I’d go to France only if she really wants to go to France. Learn the language, eat the food, love the beauty. That’s how you can take the drawbacks with the benefits.
For the student experience, I’d choose London.