What advice would you have offered this person when a guest, who was invited to her "picnic," brought a ton of food of her own?

From Facebook:

“I invited one couple for good old fashioned summer picnic fare food: Grass fed beef burgers, in-season corn on the cob & sliced tomatoes, as well as a tossed salad. Plus, cake and ice cream for dessert. Gave them menu ahead of time. No disagreement was offered.

She showed up with a very substantial “cauliflower/potato based” vegetable soup , brussel sprouts, loaf of crusty bread, wine and perrier water. And a what she pronounced as her “special apple dessert,” (recipe from her country of origin). So basically, their own dinner. Very good, but another full meal.

Meanwhile, as I was cooking & trying to serve the burgers, & the corn & salad (under our grape arbor outside), she is heating her soup inside, asking for bowls & spoons for the soup. She serves her soup crusty bread and brussel sprouts, all which (again) are very good, but filling while my burgers & corn went cold.

I feel like she co-opted the meal. I love them dearly as friends, but this has ticked me off and I am so frustrated I want to say something which makes me then feel petty and somehow ungrateful. Drat!”