What is the ruling on praying behind someone who cursed Allah during the day in Ramadan and completed his fast, and is fear an excuse for him?

The statement that an apostate is considered a disbeliever does not depend on a ruling from a Sharia court. Rather, whoever curses Allah the Almighty or His Messenger, or throws the Holy Quran into filth, or commits any of the nullifiers of Islam knowingly and willingly, is ruled to be a disbeliever and an apostate from Islam. The Sharia court's role is exclusively limited to seeking repentance from the apostate and enforcing the prescribed punishment upon him. One who curses Allah the Almighty becomes a disbeliever and is not excused by ignorance, as the prohibition of cursing the Lord and the obligation to venerate Him are known to all. Whoever rules that an apostate is not a disbeliever except by a judge's ruling has erred, but this does not prevent praying behind him or seeking his fatwa if he is among the people of knowledge who meet the conditions for issuing fatwas.