You need to ask yourself (or you mom) how having a DAS will help her access the attractions? How will does her condition make it difficult or impossible to wait in the standard line. That's what you need to be able to explain in order to obtain a DAS. They don't care about diagnosis.
This is outside of the typical need a DAS is requested for, so I don't want to get your hopes up. I understand your reasoning for asking for it, but I think Guest Relations will have to answer this one.
As mentioned above, explain her situation at GR. The DAS is based on need, not diagnosis. They don't have a list of approved conditions they are looking at.