Depends on a lot if things but yes they wont last as long as someone with type 4 hair. If her hair is on the thinner side and its actually less than 3b (so many people overtype their kids hair) then braiding it will cause more harm than good esp if left for as long as possible
Yea not gonna last long enough to put that much tension on her head. She could have singles (like box braids) but cornrows are pretty much out for this hair type without causing damage
There are a few things you can do to mitigate the difference in texture. The first is to use extensions that are more like a 4b/c texture to create the grip. You can also put little rubber bands close to the root to help the braids last longer. This can be damaging so if you do this, soak the rubber bands in oil first and this helps to reduce the damage to the hair. But yes, the hair won’t last as long as kinkier texture hair, but it can still last a while.
It is going to depend but in my experience my braids slip from the root faster and get frizzier faster than friends who have more textured hair. So if I ever do braids with added hair about 3 weeks. With my hair only about a week. Whereas I have friends who can keep them in with added hair for a couple of months with maintenance (washing, oiling, etc). It will just depend.