If she is going to be doing an externship, it means that she is going to be getting paid for it and therefore was hired by the facility and passed their requirements.
Age as well as weight have nothing to do with things, and have seen many nurses have handicapped stickers on their car and have never had any issues with keeping up with the other nurses on their unit.
As mentioned, you do not know if there was any underlying cause such as that she just had surgery or a procedure done, but then again, it also should not make any difference. Working on a nursing unit is on a level playing field and there are no hills to go up and down either. Would recommend that you wait and see what she can and cannot do, you may be very surprised.