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Is it cardio or muscle strength she's lacking? When getting back into running after injury/illness I had to take it very slow just building back from square one. A bit disheartening when you know what you used to be capable of. Short, frequent low intensity sounds better than trying for high bursts of activity, but I don't know anything about dancing training.
A bit off the wall, so feel free to ignore, but might she do some mind of dancey workout routine? Obviously taking it at a gentle pace. There's lots of free ones on Youtube (I've just done a couple by Tiffany Rothe), but I'm sure there must be others. She might like the occasional change from going for a walk.