I was talking to this girl, everything going well, then out of nowhere her friend comes over and says

"remember, you have a boyfriend missy!"

didn't know how to react, her attitude instantly shifted, she wasnt talking as much as before.

What could I have said or done?

Not much, you got cockblocked. After that, she felt guilty talking to you.

What you could've done is start joking about it. "You got a boyfriend?! You liar, you told me we'd get married!" and laugh it off with a line of jokes and close quickly before she changes her mind about you.

Beetle's best post?

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I'm a winner - I am going to win.

Always improving

Now trying to bring you down, but that's try-hard and conveys way too much AFC attributes. Honestly the best way to do is maintain the conversation you're having, because her having a boyfriend is irrelevant when it comes to talking to another human being. Let me give you an example:

Today, while I was working at the restaurant (I'm a waiter), I was building a lot of rapport with my guest. Everything was going extremely well--I was asking many comfort building questions, teasing, etc. Then at a point she mentioned that she had a boyfriend (we were discussing food envy when ordering something at a restaurant, and she was like "my boyfriend always gets food envy of what I order." So I don't get the impression of her raising a shield). I just continued the conversation normally and said "all the girls I bring out to dinner to always get food envy! I'm glad someone else knows how I feel." But the most important thing I can say here, "don't break rapport just because she mentioned she had a boyfriend." Only a jerk does this to someone. Anyway I finished up the meal with her by saying, "honestly the next time you and your boyfriend come in, request me. It'd be a pleasure to serve you again, you're fun. Also I didn't catch your name, oh that's unique! Now for the next time I see you, don't answer now, 'tell me what your idea of a utopian society would be.'" After that I dropped the check, when I returned to pick up the check, her number was inside.

So be natural, be unaffected by the fact that she has a boyfriend. "Boyfriend/Girlfriend" is only a label anyway. Besides like I said, why should she stop talking to you since she is taken, that's irrelevant. Are we not allowed to have friends of the opposite sex? I think you know the answer.