Don’t beat yourself up about it. Part of staying healthy is being able to enjoy life, and not saying “I will NEVER eat (this or that) ever again.”
The longer you spend fixing past mistakes, the worse you’ll feel about them, which can be really discouraging and depressing. Second, if you get into the habit of “fixing’ mistakes, you’re more likely to make mistakes in the future, saying “I’ll just work it off later.”
Don’t kill yourself trying to make it up with huge caloric deficits or lots of extra exercise. I looked it up, and a Wendy’s ¼ lb. burger is 470 calories. I don’t know what the nutritional information is for the new fries, but let’s be generous and assume they’re 530 calories to round it up to 1000 calories for lunch. That’s not even a third of a pound. Chances are, ONE “splurge” like this is not even going to make a difference on the scale. You may see a slight jump but that’s just water weight from the high sodium (especially in the fries) and that will go away with a couple of days of normal healthy eating.
Just get back to normal as fast as possible, eat what you’d ordinarily eat and exercise as much as you ordinarily would. If you want, you can spread out some extra exercise over the next couple of weeks to burn off 200 calories.