Totally agree with PPs on the need for caution. Without knowing her specific dietary needs, you really need to make sure that ANY ingredients you use are clearly labeled "Gluten Free" and not just wheat free. For that reason, I'd go as simple as possible with the meal. Also, I'd recommend using utensils straight out of the dishwasher, and that you clean any food preparation surfaces very well. Believe it or not, even small amounts of cross-contamination can be dangerous for some.
We're a gluten-free, casein-free family, and it's actually quite easy once you learn your way. However, for someone who is just volunteering a meal (very kind of you, by the way!), it's still easy, just requires a little extra vigilance.
My family loves a roast chicken on Sundays. We eat VERY little meat, so the Sunday roast chicken is a treat lol. I buy a whole roaster, wash it well, and sprinkle with fresh grated sea salt/pepper. I cut up an onion and a lemon and stuff them into the cavity along with a fresh rosemary sprig. Also, I like to put a couple of rosemary sprigs under the chicken as well. I them wash and slice up 2-3 apples (no need to peel) and place them in the pan around the chicken. I mix 1/4c balsamic vinegar with 1/4c grapeseed oil (can use olive oil), and pour that over the apples and chicken. Roast at 350 for 90 minutes or until the chicken is nicely browned. If you have an instant read thermometer, it should read 165 degrees in the thickest part of the bird.
We love this meal because it's very easy to prepare, and makes yummy leftovers, which can be used shredded over a green salad, in soup, etc.
Love the peanut butter cookie suggestion! That's one of my favorite recipes (1 c peanut butter, 1 c sugar, 1 egg is the recipe I've always used). Easy, and delish!
Good luck!