As someone who went through several plane flights with an elderly parent also with mild dementia, there is NO guarantee the OP's father will sleep on such a long flight. "Sundowning" is a common effect of dementia, where as the night progresses the person becomes more anxious, not less.

Leaving aside the F vs Y debate, what happens if your father becomes agitated or confused mid flight? Has he been on any flights recently where you could at least gauge his reactions? Will he be on any special medications? I think you need to make sure the disruption on his end will be manageable for his sake as well as other F passengers.