I have a similar background, the technique being used on me concentrates partly on warm-up to loosen all the muscles in the area, including the oesophagus, together with buildup of muscle tone to improve control, and also day-to-day work to eliminate ex-centric vocalisation. I'm happier vocalising from my tonal centre routinely than from the unvoiced stressed-up vocal cords, having the advantage of no urgent demands now being made on my attention, and so I'm quite comfortably reseating my voice in a different register. The upper remains open to me, but I can now notice the choice between the forced register and the open one.

Part of my problem was a high-stress precision job which required quick reactions. As such, there was no time to "slow down, you move too fast", but now I've moved on happily I can release that. Another aspect is that voice practice has moved on so much since I learned to sing forty years ago there's no end of adjunctive techniques to help which just didn't exist then, not least in the spiritual and emotional side: fortunately I am not new to meditation, which aids no end in the sense of opening. That's something you don't mention, but which is a key to releasing those muscles quickly, as it overrides the mental control which causes the problem.

Part of what I'm doing to rebuild is not to go straight to song work, but rather to run the glissando from my natural tonic upwards and back again to ensure the passagio is unbroken, rather than breaking the openness in the way you describe. It may seem strange to talk of passagio in a female voice, but it exists, particularly in the lower voices: I first met it as a boy treble, singing in the alto register, and it became a serious question for a while learning to handle my adult voice. Eventually relaxation and confidence bridged the break, together with projective techniques producing tone outside the head.

Another trap leading to this is weak volume control: again, practice, practice, practice. I sympathise with your pupil in that my working environment and vocal power were antagonistic, leading me into a thoroughly unhealthy subvocalisation technique, which I'm now able to reject in favour of controlling the volume itself.

Hoping this opens a few doors to you from the pupil's side. The key is to discover what is locking in her psyche, and possibly to find other techniques to work around it. Warm-up for sure, and finding a better technique to circumvent the passagio, possibly also volume control.