The progression for one-legged stands I learned in physical therapy was 30 seconds, 3 reps, switching from leg to leg. 2 times a day.
1) With a finger touching a table or wall.
2) On the floor with hands out in the air for balance. (Keep them low)
3) Same as 2, but stand on a pillow.
3) Cross arms over chest.
4) Look up at the ceiling.
5) Closed eyes, head level.
6) Closed eyes, head back.
7) Closesd eyes, tilt head left/right.
Once she can stand on one leg with eyes closed for 30 seconds, every 3-4 days add an additional 15 seconds.
The goal is to last until a little bit of wobbling starts. Once it becomes easy, make it tougher. This fires up all the little muscles in the knee, ankle, & hip which is where balance comes from. Basicly you want to enhance the brains abilty to know where the body parts are. It's called proprioception or kinesthesia.
Another exercise you can do together is to stand on one leg about 6-10 feet apart and toss a ball back and forth. Start out with 3 sets of 10 easy tosses directly into the hands. As that gets to be easy, toss it further and further away from the other person's center of mass making them reach out. Add 5 more tosses each week.
Don't neglect stretching afterwards. Quads, hams, abductors (inner thigh), and calves.
Hope that helps.