I would go with a 500 or 750 watt Bafang Bottom Bracket motor and a NuVinci rear Intearnally Geared Hub.
This will give you the best combination of extremely simple operation and a wide range power envelope that is good for hilly terrain.
start with pedal assist up around 7-9... And as she gradually gets in shape, she just sets the pedal assist lower. Lower pedal assists also buy her greater range.
( and, actually, hilly terrain can increase range since you use nearly no power running downhill... I have a long downhill into town...and even turn off the battery going down...Effectively saving the entire charge for the uphill leg.
with the Bafang 750... And NuVinci gearing system, get the Bafang with the twist throttle, and she can twist the left hand for motor power, twist the right hand to adjust gear...
Oh, and you'll find you won't have to slow down for her... You 'll likely have to up your game to keep up.