Is there a device that gives immediate feedback when you're off tempo? Maybe it changes color, or blinks, or beeps, or vibrates..........
Or shocks. Like a shock collar that you put on your dog so he or she doesn't stray too far from the yard.
I'm sorry, but that truly is the first thing I thought of.
Seriously though, it sounds like metronome is a distraction for her. Maybe she's struggling with the tempo because she is distracted by a device meant to help. Some people don't do good with devices like that. I say let her find her own groove. Her own pace and then give her songs to learn based on that.