بسم الله الرحمن الرحيم
ليس التفريط فى النوم انما التفريط فى اليقظه i.e. There is no negligence when one sleeps, rather negligence is when one is awake. So there is no remissness in sleep. Yes there is negligence in the state of awakening and one will be accountable for the same.
Thus in the question mentioned above when the said person continued sleeping till Isha prayer then he should break his fast after he awakes. There is no qaza of that fast nor he needs to pay any kind of kaffarah.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.