Ski boots provide a huge amount of support for the ankle. I have a similar problem (twisted not broken) and need to wear ankle supports for just about every other sport except kiing where the boot does all the work in keeping the ankle still.
If she dont have any problems daily, then she probably wont have any problems skiing. I have broke my ancle, arm, dislocated a shoulder, but I still ski without any problems. Only problem is my head after 10-12 beers at the afterski. :-)
if she never skied the beginner groups are very tame, you hardly go anywhere in the first week, its more about learning how to stop, and getting on and off a ski lift without dropping your gloves or ski poles in the deep snow half way up reckon she would be as safe as any one of us was in beginner week, but then I am no doctor
My sister broke her ankle quite badly, can't wear high heels for any length of time, but she has worked in ski resorts for last 6 years & has boarded & skied without any problems.