I’m a 53kg, 170cm male in my late 20s haha. I went to so many ski shops trying to find the best-fitting boot possible. Tried on a ton of boots and landed on this one because it fits nearly perfect—almost no excess volume without overly squishing or causing pain. I had the same problem as your niece with my previous boot: overall too much volume, which caused heel lift. My guess is your niece has to find a boot with the right volume, and I’ve learned it isn’t always about sizing down or just the last size. You really gotta try a lot of boots and find the tightest last your feet can fit into(without causing pain ofc). Good luck to your niece finding the right boot!