Steps to try...
1. Talk her through the parts of the bike so she knows what they all do. This seems remedial and dumb, but you want her to understand the machine and be comfortable with it. Make it clear that she can't fuck up the bike no matter what she does, and that everything can be fixed. Show her how the handlebar screws can be undone to adjust the bar, for example. Be sure to explain how the brakes work, and that she can use them at any time.
2. Have her sit on the bike "to size it". Fiddle with the seat and get it perfect for her (no matter how perfect it was to start with). This way she knows the bike is "just right" for her, and she can see that if she is uncomfortable it can be changed. Make sure she is comfortable.
3. With the seat and handlebars adjusted to fit her, have her sit on the bike with her feet out on either side. Tell her to raise her feet and try to balance the bike (she can't). Show her that to prevent a fall, she just has to put her feet down.
4. Now hold the seat behind her (use your knees to support her weight) and repeat teh balance exercise, having her see how long she can keep it up without you holding it. This will help her see how to sit to keep the bike upright.
5. With her balance calibrated, take her to a very gentle slope - like the drainage slopes in a parking lot - and repeat, but let the bike roll. she will find it easier to balance as the bike moves. Repeat until she can make it to the bottom and partway up the other side. She may figure out to turn to ride the inclines on her own. Cheer loudly.
6. Get her to try pedaling, but show her that by pedaling she moves the balance point. Likely on the second or third try, she'll be riding a bike.
7. She can ride and stop by now, but not start. Learning to push off is harder, but once she has enough confidence, show her using a curb so her foot is up off the ground to start, then graduate to pushing off the ground.