Maybe next time you are invited to eat with someone else you could ask about what is to be served and explain that you need to check, just to be sure you will be able to eat it. It is not being impolite if it means that you will not feel dismay at what you are served up - I would suspect that your friend must have picked up that something was wrong even though you ate the food prepared for you.
I have eaten out twice this weekend - both times the person taking my order was rather taken aback by what I did not want on my plate - I think that the world in general needs educating about the requirements of diabetics, and it is down to the diet controlled type 2s to point out that not everyone can cope with half a plate full of heavy carbs, which seemed to be the standard offering.
By the way, the coleslaw at Harry Ramsden's in Swanage was absolutely vile - and rather stale. For a widely known chain of restaurants I expected something better, and fresher. I can recommend the Red Lion, though.