Maybe offer to bring a salad, or something like that? I'm gluten intolerant so if this sort of thing came up I couldn't eat it at all (and I'd be scared to eat anything they'd prepared because of cross contamination issues). Obviously it's not so bad for you and it really isn't at all easy dealing with these kinds of situations, the potential for hurt feelings etc, but you know the effect the carbs would have, so you've still got medical reasons for not eating it.
Or if you don't want to say anything, try to eat as little of the pasta bit as you can. The filling would be mostly fine.
Worst case, eating it and acknowledging that you're probably going to get cravings and committing to dealing with those immediately you've eaten a small portion and not letting those sneaky cravings talk you into dessert or into continuing to slide tomorrow (they will be painful, but you know fighting them is worth it). You'll probably have a water weight gain too, but if you get straight back on you won't be gaining fat.
Good luck Rho!