I think most will go along with the idea that once in a while a lapse from low GI planned diet when you are eating out is acceptable. I am recently diagnosed T2 and I will have a few potatoes where normally I would substitute sweet potato or lentil etc. In my case it is usually a Sunday lunch type gathering and I do make sure that I get some exercise later on after such a meal ( not too soon after eating obviously) and I find that my levels are fine. You have to excercise self discipline over portions though and be firm but polite when offered seconds of the bad parts. I have found most friends very helpful and without making too much of a song and dance often adjust meals to help out avoiding very fatty foods and using sweet pots themselves as a very tasty alternative to regular ones. If you feel you shouldn't have the pudding just don't eat it, try to get over the idea, no matter how deeply engrained, that the only "treats" you can have are sweet things.
The one aspect I am on a bit of a soap box over though is this concept of the "treat" when the treat involves double chocolate sticky toffee cream fudge sundae cake with cream and extra cream. I haven't touched a pudding a bit like a reformed smoker who daren't have just one cigarette, I am not putting such things in my remotest repertoire, it is just too much of a risk to my discipline. So I guess I have to admit that I recognise my own weak will and one pudding would probably lead to another, I live within 5 minutes walk of a Waitrose which is a very dangerous if my mind might wander in the direction of gorging on a Lemon Meringue pie or the like 🙂
Very well meaning people tell me that I don't have to cut out sugar completely but perhaps the shock of finding out the consequences of badly managed diabetes has made me profoundly change my "palate" and I have managed to eliminate all refined sugar and puddings, biscuits etc without it being much hardship. Maybe I am a bit obsessive but I can truthfully say that many things taste better and you realise that you were often just tasting sugar rather that the food itself, same happens when you cut down on salt.
I had a very sweet tooth and had perfected home made fudge and I could munch my way through every pudding the Gu company makes so it was a sacrifice, but one worth making to do the best for my health. So when others have pud I have a little cheese or small piece of fruit. With all the money I save on puddings I can afford smoked salmon as a treat instead, and more testing strips when the NHS ration runs out !!!!