What could it mean when a sr citizen becomes unable to eat absolutely any kind of food without becoming immediately ehausted/unable to keep eyes open?

Post meal symptoms. As we age (and get a little frailer) eating tends to have a marked effect on blood pressure. When you eat, it is normal for your circulation to increase to your GI tract to help digestion and to decrease in many other places. If we are older, on a lot of medications, and are not in good physical shape, this change of blood pressure reduces circulation to the brain and makes us fatigued and sleepy.
Blepharospasm. An unusual problem in mostly older folks is a spastic closure of the lids which actually is blinding because it is involuntary. This is called benign, essential blepharospasm. Have your ophthalmologist check this person out and sort out the problem.