what’s the best reaction when I’m talking to a girl and some guy comes out of nowhere saying ‘she’s my girl friend, even though she’s not? But he just trying to pick her up!

Like he’s just trying to ‘protect’ her or show off in front of her. That happened to me recently: I talked to a girl, then she spoke to another guy, and when I came back and spoke to her again, he suddenly came up and said ‘that’s my girl’ just to make himself look good (i know it's dumb)

But how to handle it, without appearing in his level?

What’s the best move in that situation?”