She should carry some type of sugary substance with her for just such an event. If at home, orange juice would be appropriate. For when she is out and about, try getting a package of the sugary gels that bike riders use. That or a piece of candy will work. If this is a regular occurrence, you need to discuss this with her doctor.

If she is able to swallow without coughing/choking, give her a cup of orange juice.