You're talking about a pedal with a "cage"? Get rid of that! I'm an AK, and I can tell you that something like that would give me fits. She needs a regular pedal.
I don't remember anybody helping me learn to ride a bike. I struggled with it, and even tried to put the bike in the garbage can, out of frustration. I guess I was about seven years old. Eventually I got it.
I start by placing my prosthetic foot on the pedal, sitting on the seat, and I push off with my real foot. This usually requires me to push two or three times. I never get enough momentum from a single push.
Maybe you can get her to sit on the seat and push herself around with her good foot, not trying to pedal at all. When she's comfortable with that, she can move on to pedaling. But her prosthetic foot should be firmly on the pedal before putting the bike in motion... or at least that's what has worked for me, and I've been riding a bike with a prosthesis since 1966.