Well, she might be wise to get some help and file a timely complaint thru EEOC.
The latest version of ADA coverage post about 2008 covers a very broad spectrum of limitations.
If she was fired for poor judgement and giving away food--that's one thing.
But if she had learning disability that inhibited her ability to count, deal with cash registers, make change --and there was some sort of recognized need AND some sort of accommodation made to accommodate her limitations , in this case by removal of her from register duties and maintaining her at cooking only duties --I think she has a point well worth review with competent labor counsel. Its not a slam dunk either way!
I have no quarrel that management might not have a need to change the accommodations--and a right to do so --but if they just unilaterally changed the rules--I'm not so sure that fits.
The issues of the mentally challenged are perhaps most vexing.