She's tried retraining it about 6 times, at my insistence. It's just crap. I've since paid more attention to other iPhone users I know, and they seem to be doing a lot of passcode entry too.
Does she look at the camera while unlocking? It optionally requires your visual attention to successfully unlock. Otherwise your experiences are completely at odds with our experiences. I’m family tech support for nearly 15 iOS users and none have this experience.