I think many people have to be tranquilized, so your wife is not alone.
Tell her to visualize the tube as just that, it's not sealed at the ends. If all else fails she can wiggle out.
We have a close friend in the UK that wouldn't face the narrow stone staircase to the top of the church. She is profoundly claustrophobic.
It was a long climb, with shoulders brushing the sides. The Rivetess talked her through every step and when she came out onto the roof the rewards were huge...a fabulous view. Then there was the return, but she made it just fine.
I don't know if someone can remain in the room and hold her hand. That would help.