How can I eat healthy at friends' houses?

Okay, so this is what always happens. I will lose a lot of weight, but then gain like half of it back whenever I go somewhere for even a full day. It is soo irritating... It stops me from hanging out with my friends... My best friend eats really unhealthy and her family eats dinner late and has a loot of it. How can i stp myself from gaining weight if I don't have any healthy choices? Does anyone else have this problem?