This would be an ideal situation for the disability card. (PDF at link.)

My Mom had Alzheimer's but we never tried air travel. It will depend on the level of disability, of course. If she can follow simple instructions and is not using a wheelchair, it should not be too bad. I suggest that if presented with an option, the AIT may be the best of a bad situation. She may not understand the pat down, and with a chronic medical condition the dangers of the scanners may be less of a concern.