How old? that helps determine what is appropriate for maintaining muscle strength. It's also hard for a "Board" to give you recommendations, as we don't know the type of break, where it is, what is contraindicated, etc. I might suggest swimming ... but, if that pulls on the leg in the wrong way, it could mess her recovery up.
A physiotherapist can certainly give you a range of exercises that will build and maintain muscle strength, and should be your first line of defense; plus, it will give you a good working relationship for when the cast comes off, as you will certainly need a physio then.
My kid sat for 6 weeks in a full leg cast; got back on the ice, and took about 6 to 9 months to recover lost ground with weekly intense physiotherapy. We did physio and training while the cast was in place, but, also took the time to rest and do mental recovery. However, if we had not involved the physio from day one, I know (and the orthopod confirms, as do all of the sports medicine experts and experienced coaches), the recovery would have been far longer and far less successful.
Don't let the length of time it took my kid to recover discourage you though: they're all different, and we also had to compensate for a growth spurt during the sitting period, which would have slowed us down regardless. Also, having to go back and slow down things allowed for a focus on singles and basic skating which have certainly paid off in the long run.