What's your average speed?

check out the same thing from lunacycle.com if you want a mid drive. I own one.
$400 Bafang motor. Add your battery.

Here's a Dillenger front hub motor. Tradeoff is you want a steel fork for it, not alloy.
Dillenger 700cc for $629. Comes with 36V battery,

There's lots of 1000W direct drive kits on ebay for $200, but the problem with them is too heavy, too much power, and not well set up for pedal assist. I think she would have a hard time going slow (under 20 mph) without the electricity taking over the bike, Also, they're ugly, and hard to pedal unpowered. Good for going to work at 28-30 mph until you crash.

I think the best kit is a rear geared hub motor, about 350-500W, with pedal assist and throttle. LCD display. Ability to run on 36V, 48V, 52V. Those are what I have on my bikes. They're cheap and reliable. Will loaf along all day at 14 mph. Over 22 mph on 48V. Pedals like a normal bike. About $200-250 for the kit. Trouble is, no one sells them in the USA anymore. I had to buy them from China.