Mobility or stamina issues are encouraged to use a mobility device such as a wheelchair/
ECV/rollator, and DAS is usually not issued.
OP - technically, waiting outside the queue and then joining the rest of your party would be considered "line jumping." But in today's world with social distancing, many queues are stretching well out into the walkways so she may be able to join you near the building. What may work is for her to be with you when the party first joins the queue, then say "I'm going to sit over there and will meet you when you get close to the building." (This generally allows others near you in line to know she is there and will be returning.) However, be aware there may not be any place nearby for her to sit and wait, so she could just be standing somewhere. That is why Guest Relations will suggest a mobility device because they can't guarantee a place to sit while waiting. And as
@lovethattink mentions, most queues are slowly moving and not simply standing still for long.