When your new friends, who also happen to be self-professed lousy cooks, have invited you to their house for a meal, it's time to strategize.
At mealtime, first and foremost, do not bring up the quality of their cooking. Instead, plan ahead: Approach the situation with awareness, knowing you probably won't be eating a full meal and eat a snack before and bring one with you for after. Make an effort to at least try a small portion of everything that you're served.
And for the next get-together, strategize some more: Offer to bring a dish, make the meal a pot-luck, or even better, cook dishes together and share recipes.