That would not make her un-guilty of disorderly conduct. I disagree. The employees could make the report at leisure instead of inconveniencing all the innocent customers. Suppose she had simply left after sitting there ten minutes but before the cops came. Wouldn't that be the same position if your argument applied? (Which it does not.)
You really think they would charge her after the fact?
So, you think it should have happened like this: lady cuts in line, McDonalds says "Don't do that again!" but serves her anyway. Lady leaves because she was served and McDonalds is supposed to call the cops and say a lady took cuts in the drive-thru line, here is her license plate number. I wouldn't think the police would spend 2 seconds trying to find this lady.
You are going to great lengths to appease and enable her bad behavior. Call the police after she leaves? Yeah, that’s going to be their high profile case for the day, after they get up off the floor from laughing.