I guess I'm a little lucky: My FIL is type one, so he gets a lot of this stuff.
I think that this might call for a little "GD deprecation" as in: "oh man, I'd really love to eat that, but this GD diet is sooo messed up that I can't. Isn't is dumb that I can have XX but I can't eat YY?" Then I'd apologize and go get my own food.