I'd doubt it very, at least not long-term. Generally, you do need some strength and mobility in your ankles for riding (moreso in English than in Western), but not to the extent that you would need for high level skating. Mostly you have to be able to bend the ankle so that your toes are up and heel down, be able to apply pressure with your calves, dig your heels in and stand up in the stirrups with your ankle flexing and supporting some of your weight. That toe up-heel down position does stretch the back of the ankle, and that might be problematic right after the injury, but I would think that you would be able to adjust in the long-term, even if you lost some flexibility. It's fairly common for riders to keep competing with leg injuries, such as broken feet or toes (obviously not the same, but you can safely adjust to foot/leg issues while riding at a pretty high level). I know someone who got back to riding everyday after a broken kneecap (she wears a brace on it). Riders do require strength / need to be in shape, but because it's physically a partnership with the horse, it can be more forgiving than some sports in my experience.
You can also always do bare-back or no stirrups riding if you have a good horse, which would put almost no strain on an ankle, except for getting on and off.
Anyways, if she's back on the ice now (it sounded like she is), then I would guess that she's good for at least light riding.
Sorry for being a bit off topic - I was a competitive horseback rider, and I can't help going into too much detail whenever horses come up.
