Rule #1. Find a way to make her feel part of a
happy crowd.

Check with the C.F. foundation and ask about any dietary needs.

Ask friends and family what she likes

Make the time about everybody and make sure she is part
of everybody.

Dark chocolate is especially good, but I'm pretty sure she can eat most anything as they function better with a high calorie diet.

https://www.medicalnewstoday.com/articles/326511.php#dark-chocolate

Whether eating at home or away, here are some simple tips:
• Drink whole milk and milkshakes.
• Add extra butter or olive oil to foods like potatoes or pasta.
• Use regular (not diet) dressings on salads or vegetables.
• Eat burgers with bacon and cheese.
• Eat pizza with extra cheese.
• Add cheese to sandwiches.
• At breakfast, eat omelets with extra cheese and ham or bacon.
• Top salads and sandwiches with avocados or guacamole.
• Eat calorie-rich desserts such as ice cream, pudding, and cheesecake.
• Top hot chocolate, pudding, and other desserts with whipped cream.
• Avoid diet foods – anything that says nonfat, low fat, reduced calorie, or light/lite on the label.
Besides eating high-calorie meals, it's a good idea for guys and girls with CF to carry some high-energy snacks with them; try trail mix, nuts, packets of cheese crackers or peanut butter crackers, and veggies like carrots or celery with small containers of dressing that don't need to be refrigerated.
• https://kidshealth.org/en/teens/cf-diet.html