Do you really care if someone is a PUA or a natural ? Why should that be important ? Natural or not, he was trying to steal your target. You should have tried to lower his R&S value by negging him in front of the girl so that she can see that you are more alpha male than him. Another thing you could have done is just to open another hot girl, start gaming her. If your target sees this, she will probably get jealous and leave the AMOG in order to win you back. Or you game another girl, than introduce her to your target and the AMOG and suddenly say to her: "OK, let's now leave these 2 alone because this guy is really doing his best to pick my friend up and I don't want us to spoil it for him

. " Look at the guy and tell him: "Good luck ey

". By doing this you ruin his game. Also, the target will feel more attracted to you because you walk away with another girl and you don't care that another guy is gaming her.
In this case, he used a canned line so he probably isn't a natural. I think if a guy is doing 90 % of the things you read about than he isn't a natural. If you see him negging, kinoing a lot, tease, push-pull, some routines that look like canned routines, ... than he probably is a PUA and not a natural. I also wouldn't bother to ask him about it. Especially not if he tries to steal your girl. I would use my knowledge of PUA to dominate the frame and make him go home crying.