Alex, I don't think that anyone could give such a number (i.e. for those who are neglecting they have diabetes) - nor would you find those persons on a site like this.
And, YES, there is a strong correlation between constantly high HbA1c and diabetic complications.
There is a difference in strategies, however, between the way persons with T1 or T2 handle their conditions. So maybe your T1 friend is in good control - even with drinking coke. Why don't you ask him how he is managing? If he is a friend, he surely wouldn't mind if you ask him in the spirit of interest and wishing to obtain information rather than blaming him for his coke-intake.
annelise