It is possible that your friend had GD that wasn't picked up, or that she had a borderline hyperglycaemia that affected the baby once he was out of the womb. However, being large for gestational age is an independent risk factor for low blood sugar, so even in a non diabetic mother, large babies are at higher risk for this.

My understanding is that this is because they just have a higher metabolism and so burn their glucose and glycogen stored more quickly than a normal sized baby.

We had to screen all babies over 4kg and under 2.5kg regardless of maternal diabetes when I was working on the nurseries. Hope that helps!

Hope he does well and it out and with his mum soon!