Even in the unlikely case that someone is IDBed on the flight, it's unlikely that this would happen to a handicapped passenger. I'm not sure about DL, but most carriers have in their CoC rules about priority for IDBs that handicapped revenue passengers are protected. if you're worried, check the DL CoC for IDB rules and make MIL aware of the essential points that apply to her, including the requirement that the airline first look for VDB volunteers.