Hello,
It sounds like your cousin may have had a seizure. She should see a doctor, a neurologist would be good to find out why. It can be caused by different kinds of medical conditions which may need to be treated. They may do a brain scan to check for tumors. Often they do not find a cause, called epilepsy. Medication can be given to help control it.
When it happens it is important to protect the person from injury like if they fall. It is good to turn the person on their side so if there is fluid in the mouth it will run out and not choke on it, do not try to put anything in the mouth. Sometimes people will sleep awhile afterward and can't remember it happened. It is frightening to see but will usually pass in a minute or so. Do get checked out and good luck.