there was a problem like this once, simultaneously was very itchy around my lower back and legs, was very very thirsty and could not derive the will to either drink water or scratch, it was absolutely hell, but after i finally drank some water and had a shower it disappeared.
this was from an aya mind you, so its a bit different, and given everything that was going on and that i did not actually scratch despite how bad it felt i cannot subjectively say what happened.
it sound though that this was a stimulus reaction, put her on aya or lsd and she could be so far away from her body she wouldn't be sure if she is scratching her arms or the a three eyed cat.
my advise get some aloe or menthol or eucalyptus or any mild numbing agent, if it happens use it, or take a shower.
you could also do a a scratch test and check if she is having an allergic reaction, put a drop of what ever your checking on her back then take a small pin and scratch her with it inside the radium of what ever your testing (less than .5mm, don't jab her just expose some of the lower skin)
but don't do the scratch test WHILE TAKING ANYTHING

i think she will be fine, just take your time and make sure she wants to do it and start the doses low