Just treat it as if she has no bf, and if she brings him up in a conversation, either ignore it or use a bf destroyer.
This is one I got on another seduction board and is pure genious, credit goes to Romeo7 from that board.
HB: I have a boyfriend
PUA: I'm glad that you're straight, If you were a lesbian I would prolly be hitting on you
HB: (CRACKS UP)
PUA: Let me ask you something, Look at me, and tell me, do you think I would make a good boyfriend?
HB: Yes/I don't know/ Maybe
PUA: You see, I bet you your boyfriend is like the greatest boyfriend in the world, he probably calls you 5 times a day to make sure you're ok, to see where you are and with who, buys you roses and chocolates.
HB: Not really...
PUA: Stop hiding it... You guys probably make a great couple, and with no doubt will be married in a couple of months and have handsome kids
HB: I'm not even thinking of marriage
PUA: Yes, you are,
**** ALL OF THAT WAS TO DEVALUATE HER BOYFRIEND'S IMAGE****
**** BUT IT KEEPS GOING *******
PUA: Anyway, getting back to my question,I think i would make a terrible boyfriend
HB: Why you say that?
PUA: I wouldn't call my girlfriend five times a day, I would call her ten (isn't that sweet) *** she laughs**** ok, eleven.... Then I would make fun of her friends and have her cook for me, and finally I would break up with her on Valentine's day
**** ALL PLAYFUL TONE BUDDY *****
HB: HAHAHAH, you're so mean
PUA: Yes, I am
***** HERE COMES THE TRANSITION TO SEDUCTION *****
PUA: So, let's go to eat, you can bring your boyfriend if you want i would love to meet him
HB: really?
PUA: no
HB: hahaha
PUA: we are going to eat, as friends, but of course you're paying
***** THIS IS SOLID, THE "AS FRIENDS" DRIVES GIRLS MAAAAAD ABOUT YOU ******