hi, Tonic clonic seizure
or for that matter any type of seizure is due to increased voltage or current in the brain
which occurs suddenly because of brain cell membrane
wekness or problem. so medicines are given to correct it. It is 96 to 98% curable after 3-5 years of continuous treatment with optimum dose of the drug with 4-5 years of fit free period.
There are various stimulating factors which cause the voltage in the brain to go up and one of it is sleep, so your friend is getting seizures only in sleep which we call nocturnal generalized seizures. Please meet a good neurologist
and get proper and early treatment as early the treatment the better is the outcome in epilepsy