I hope she gets off the hook... it's not going to be easy if they decide to take the case on, they could say she probably was listening to something (it's not necessarily a phone call) and it will be her words against theirs.
AFAIK though, in a case like this, she has to prove that she actually didn't do the deed. A statement from her mobile phone company should suffice.
IMO, if this happens to me, I would just show my mobile's call register to the copper, so he can see that there's no incoming/outgoing call at all - that should convince him enough. And if it doesn't I will ask him to impound my mobile to be used as evidence against me (which I know won't work, as phone company record will show no incoming/outgoing call at the time). As I will be going to court.
Speaking of phone record, does your sister get monthly phone bill? My bill shows all call details... but it is a bit hard with text msg though.
Good luck with the case