It's a bit of a red flag that she'd trick you like that, but it does raise a point about you - why do you feel uncomfortable with her meeting up with someone she used to know?

I think you need to take a long moment and think about why you reacted like that in the first place. Forget your gf for a moment and her weird prank, ( she's a whole seperate issue that needs a therapist)- it's not normal to feel jealous over another guy in someone's life who your other half used to 'hang' out with.

So reflect, your gf chose to be with you, right? So why don't you trust her with an old friend? Unless she's cheated before, what are you basing your reaction on?

As blunt as it sounds, you have no say in what other people do at all. You make your choices and they make theirs. You essentially telling her to not see someone because you don't like it is an attempt to control someone else's choice. You need to figure out why you reacted like that before you go any further because you then become the massive red flag to a woman.