What Causes Sudden Decrease In Sugar Level After Eating A Lot?

Hello. I'm teresa from malaysia. I'm asking on behalf of my dad, 66 years, 155cm/+-80kg and have asthma since small and diabetes for more than 30 years. His sugar level has always been on the high side despite the fact that he has been on medication and insulin jabs as he does not really control his diet. 2 weeks ago, he was discharged from our local hospital after being admitted in there for 2 months. He was diagnosed with Chronic Kidney Disease. It has already affected his kidney, heart and particularly his lungs as his body is unable to get rid of the fluid and water has entered his lungs. While in there, he has had 2 times of peritoneal dialysis on which the 2nd time, it develops a bacterial infection and he had to be isolated. But they manage to get him into a better condition for him to be discharge 2 weeks ago.. Early yesterday morning, my mum tried to wake him and he's not responsive and foams was coming out of his mouth. We had to get an ambulance to send him to hospital again.. The medical officer that came tested his blood and it turns out his sugar level is only 1.6 which is very very low.
I would like to find out why is it that after my dad was discharged till to date, he couldn't stop eating, and we had to stop him so his sugar, potassium and etc is in control. But why low sugar level instead of high?