Mid-drive system seems to be preferable for hilly regions since it coordinates with the gears to provide more optimal climbing ratios. A big rear hub motor is also an option, but not as good as mid-drive IMO. Either a Bafang (8 Fun) BBS02 conversion if you have a donor bike (look at Luna Cycle) or OEM bike (Haibike, Specialized, Trek) would satisfy her (I think).