Hi there,
I have Temporal Lobe Epilepsy, and the first time I had a seizure they sent me home because the doctor said many people have just one seizure and never have one again, so they had to wait and see if I had a second one. Even after the second they were reluctant to diagnose me with anything. Eventually they did though. Also, I have had scans done where nothing abnormal was shown. Sometimes things can be missed.
I agree with the above posters...if she used drugs, that could definitely cause a seizure. If it turns out that she didn't use drugs, and keeps having seizures, definitely take her to see a neurologist ASAP so they can figure things out.
I hope you find out what caused these seizures as soon as possible!