She was probably waving her hand to dismiss his attempts to pay, or his thanks or protests. It means it's OK, don't worry about it, stop discussing it. It's not important that it's in front of the face - that was probably just a coincidence.

As Claire said, it's not any sort of 'standard' gesture. It's just what one person did in a certain situation.

Interesting post! oddly enough, im having a bit of a hard time understanding the gesture. However, given the context provided by OP and some of the commenters, im gonna take a guess - The hand was probably waved in front of the person's own face (in this case the mcdonalds worker) and NOT in front of the customer's face. Now that I think about it, this gesture is something Ive seen a lot ok tiktok. All that being said, I conclude that its like others have said - a gesture that means "end of discussion"