Onion rings are not bad for diabetics when eaten in moderation.
As long as the diabetic does not eat too many onion rings the onions will be safe for the diabetic to consume.
Eating too many onion rings when you're diabetic could spike your blood sugar or blood glucose levels so make sure to keep an eye on your blood sugar and blood glucose to make sure the levels do not get out of hand.
If you notice your blood glucose or blood sugar levels increase then cut back on the amount of onion rings you eat and you should be safe.
My mother is diabetic and she eats onion rings a few times per week and her blood sugar and blood glucose levels have been okay.