Paris, for sure! It sounds like the most immersive and also the most “different” thing from what she is doing right now. She may have an opportunity to do things in London later, grad school or not, but living with the host family in Paris is the most immersive thing to do. Just be careful how her school credits align toward the graduation, curriculum is different sometimes, and every school has their own grade conversion system.
S15 had his Study Abroad semester in Hong Kong’s HKUST and had lived with the host family in Japan when he was in elementary and high school (same family both times, it was amazing), those were transformative experiences.
Also - fall in Paris is truly special! London is also a short trip from there to visit though!
Good luck!