I would suggest it is recorded on her health form or in your first aid log the same as any other incident that needs attention or treatment. That then needs to be kept for whatever the advised period of time is.
We regularly have this with some of our Guides who are epileptic. Only need official forms if she has been hurt as a result. Otherwise just record time, date and length of seizure on back of health form and keep forms for the recommended amount of time.