Let my friend pay for my food

I’m going to hang out with a friend. She will drive me to the park and we will walk around. She says we will stop by a drive thru restaurant to get food. I don’t want to buy food so I will bring my own food. However I will let her buy me food if she wants. So is it appropriate to ask this question when we arrive at the restaurant:

“Are you going to buy the food for me? If not I won’t buy it. I have my own food”

or is there anything firmly assumed here. Like obviously, where she said so, she means she will buy food for me.