I would suggest you look at post #6 of the disABILITIES FAQs thread, which is about Guest Assistance Cards. That thread is near the top of this board or you can follow the link in my signature to get there.
They are NOT disability passes, just a way to more easily let CMs know what your needs are related to a disability. they are not given out for a specific condition or diagnosis, but are based on the needs that person has that are related to a disability. Different people with the same condition will have different nerds. Some may have problems with fatigue, walking/standing and only need to use a wheelchair or
ECV in line; you don't need a Guest Assistance Card to do that.
She will need to go to Guest Relations and explain her needs - just knowing breast cancer won't tell them that.
And, the cards are not meant to be front of line passes or shorten the wait in line. Sometimes, the wait using a Guest Assistance Card will be even longer if there is a specific waiting area that is already full or there are the allowed number of people with special needs already on the attraction ( the number is often for safety and evacuation reasons).