Anything under 90% on the pulse oximeter is low. She should go to the hospital.
That’s what I have read as well. GET TO THE HOSPITAL! IIRC you can handle less than 90 for a little bit (an hour or two???). Of course, it sounds like she is continuing to go lower which becomes even more critical.
Prayers up.
Took a sleep study test for low oxygen. Not snoring. They found my oxygen level dropped to 79 when I slept. I even use it sometimes during the day when I am relaxing. She needs to go to the ER.