Something like this?
View attachment 930990
Technically folding/portable chairs are not allowed. Neither are backpacks larger than 24" x 15" x 18" which may be smaller than the size of the stadium seat worn as a backpack. What would she plan to do with it when not using it? It may not be allowed to be taken on rides or into restaurants since it's kind of big in "backpack" mode.
Would a more basic seat cushion work for her? That is more likely to be allowed than the full seat, though it still cannot be used on most attractions.
View attachment 930997 or maybe
View attachment 930996