It's obviously scripted.
The chick's reactions to the intruder aren't what you'd normally expect in a real life situation.
Assuming the dude walking up is cool enough and the girl is giving him some level of interest, the best strategy is not to buy in the other dude's frame.
He'll try to befriend you and make it seem like it's perfectly normal what he's doing.
You should not buy into his frame, while minding the law of least effort.
If he totally ignores you and engages the chick, you gotta sit back and look bored. That should exert social pressure on the chick to cue him out of the conversation.
If you've got a half decent connection with her, she's not gonna flirt with him like you saw in that video.
If he engages you, no what he says or does, whether it's a handshake or a "heyy brother", I'd say:
"That's a weird thing to do" in a perfectly detached, almost amused tone.
Let's say he defends himself by framing what he did as friendly.
Me, inquisitive look: "Being intrusive is friendly to you?"
He'll keep defending himself.
You keep casually challenging the frame.
Framing him as "weird", "intrusive", "unsmooth" all does good things for you in the eyes of the chick.
Best, given that he's anyway exerting more effort than you by coming over, you can frame him as try hard.
"Props to you for trying buddy, that was some good effort, I'll give you that"
I must say tho, any guy who's sexy and smooth enough to steal a chick from you while she's on a date would never try to hit on her while she's with you.
He'd be socially aware enough to know that it would put him in the chasing position.
But yeah, the gist of it is social pressure.
If he doesn't engage you, you apply social pressure on her while looking bored.
If he engages you, you apply social pressure on him by challenging his frame.
Watch out for compliance demands. These are easy ways for him to get you to buy into his frame.
I'm curious what others have to say about it!
Any answers other than "befriend the dude" would be interesting to explore.