How do you prove that they did leave the line and are not just catching up with their group? You can't. Hate me if you want, but I still won't allow them through.
I don't hate you for it, in fact I couldn't really care less. If anyone thinks that I am going to waste my whole vacation worried about one or two people getting ahead of me in a line, they need to think again.
On the list of possible human problems it is on the very bottom, right next to cleaning the toilet. I have always believed, for myself and to teach my children that there are people out there that don't care one little squat about other people. It is always me, me, me! And it always will be. I place a certain standard on how I live my life and what I can do that would make me feel good about myself. So, I would never cut in front of anyone in line, intentionally and I wouldn't waste one minute of my time worrying about the behavior of others.
If Disney doesn't care enough to enforce their own rules then I have no reason to worry myself about the actions of others, within reason. I never said that I would let a large group by me without considerable protest, but, since, as you say, I cannot prove that an individual wasn't there already, I am not going to give even a minor amount of concern if an individual goes ahead of me. I may not approve, but, I am certainly not going to get all in someones face about it. Chances are that if it is one individual or an adult and child, that they are just catching up to their party. With numbers like that I am not going to get on the ride even one second later then I would have if I hadn't let them go by because they will probably be riding together. Either way, the same number of ride vehicles will be used and I will have gained nothing by making a big deal out of it.
This whole thread started out by the woman involved saying that an adult pushed her daughter aside and stated that they were going to slow. Maybe they were, maybe they weren't. Irregardless of that fact, he pushed and made contact with another person, didn't ask and just pushed his way through. Now something like that needs to be reported because it is going to happen again. And if anyone in Disney is listening, YOU NEED TO DO SOMETHING ABOUT THAT TYPE GUEST.
If we are going to get all upset about that, why not get violent about the biggest tolerated line cutter in all of Disney, the famous Fastpass.