That has always stayed with me, and is worth thinking about in all of these cases. Sometimes we should, sometimes we shouldn't. It's not easy and is a decision that should never be taken lightly.
It’s true the Church teaches no one is under obligation to take extraordinary measures to preserve life. The nun you spoke to was correct.
It sounds like, sadly, Mr Kasum is in such a state. It would be best if those who know him best (and don’t stand to gain anything) be the ones to make such a decision now.
It seems the daughter will gain nothing by her father’s death. If that’s true and his wife gains by keeping him alive (I don’t know if that’s the case), then that seems to be the answer right there.