Ahsan Allahu alyak, my freind's family is deobandi, as a person who is still in his dad's guardianship, what should he do? He is a very strict one and hates salafiyaah a lot. he is not aware I left this devieant sect, He is using my account aswell to study ilm .

Ustadh AbdulAziz al-Haqqan

He doesn't need to go public or tell them. He can protect himself from any harm, worshiping Allah (Subhanahu wa ta'ala) based on the Quran and the Sunnah, upon the way of the Salaf as-Salih. He should depend on Allah, make du'a, and continue upon goodness.