I spent a year in Paris as a student with a good command of French. It was hard but very rewarding. There are some rude Parisians (sorry Parisians but it’s true) but it wasn’t enough rudeness to spoil my experience. There are also some super lovely Parisians that will make your daughter’s experience worthwhile. There are also MANY students that are not from Paris. I made lifelong friends from the South of France, Austria, Switzerland, Romania, Spain and China. This was a while ago. Last time I was in Paris for a conference I was pleasantly surprised by the friendliness of my random interactions with the people of Paris. Maybe things have changed and there is more acceptance of us foreigners? I don’t know. I can wax lyrical for hours about the city itself, the arts, the food, the music, the cafes, the hopping on the metro and exploring a different arrondissement every weekend, trips to the seaside, Mont St Michel. It was hard but fabulous!!!
London is also lovely but I would recommend the “stretch” experience (London will be a cultural stretch but not so much a language stretch). Your daughter might feel otherwise! Both will be a great opportunity for her. I wish her luck!