She could not and would not break out of a properly constructed stall. Such a stall is framed with 2X6 and lined with oak to kicking height and pine above that. The door needs to go top to bottom and be solid oak. Stall can be solid or have bars.. but the bars need to be really good.
I had to build stalls on my farm that would house a 2,000 pound bull and I did.. and he tried but could never break out.
I had plenty of horses that preferred to be out and mostly I kept my horses out... but I taught them all that there were times they had to be in a loose box because there may be times in the horse's life that you must confined them (illness, injury and so forth).
I also crate train and outdoor kennel train all dogs.. again.. not because I am going to imprison them for hours on end but because it is a skill that may come in handy in the case of injury recovery and so forth.