Of course all that begs the question; if she's so involved with her BF why is she sitting there partying with her friends, on a GNO, or otherwise putting herself into a position / environment where a guy might approach and open her? Why isn't she with the BF now, or at least planning her next step with him instead?
In the back of your head, this is the subconscious association you're confronting. That's your point of confusion when you start a thread entitled "Girl tells me she's got a BF in casual conversation WTF?"
So how do you deal with it? First you have to know what it is. The Boyfriend Disclaimer is a preemptive LJBF. The Boyfriend Disclaimer rejection occurs when a woman slips the information of her unavailability (due to the BF) covertly into casual conversation. It's simply a communication methodology with the latent purpose of preemptively rejecting him (where a LJBF is a reactive rejection). Men have a tendency to think women are abstract and scattered when in fact they are very calculating, it's just that covert communication is second nature for them.
So what's the counter to this? Should you just give up on the approach? Possibly, it's at this point that you need to reassess your interest in the girl. If you've gotten to the point that she's put out the BF Disclaimer, most likely you've telegraphed your interest in her, she's picked up on it and covertly offered you both an OUT of the approach. Let me repeat that, if you get a BF Disclaimer she's picked up on your interest in her. This is of course your main failing. If she were interested in your advance, attracted to you, etc. you wouldn't get the disclaimer, boyfriend or not. If you are seriously a better option for her, a woman wont actively self-defeat her interest.
But you got the disclaimer so what's next? You need to evaluate whether the girl is worth the effort first. She's not where she is at the moment because she's so end-of-the-world in love with the BF, assuming there is a BF to begin with. That's the first assessment. My best advice is to base this on her IOIs with you, her covert interest signs, buying temperature, etc. Every girl can be sarged, but not every girl is worth that effort. Women who use the BF disclaimer (like the LJBF) have more likely than not used it with success before, with predictable results. So the counter to it is to use the unexpected. A BF destroyer is one way, but I'd suggest not to go with something so scripted. C&F is great, something along the lines "wow, your BF sounds great, what's he doing with you?" will not only neg hit her, but also overtly draw attention to her use of the disclaimer.
One very entertaining response is using direct game with a BF disclaimer, and basically call her bluff. You'll have to have brass balls to pull it off, but by directly calling her on her disclaimer bullsh!t, you force her to acknowledge it. This response is akin to telling a LJBF girl "no thanks, I have enough friends". You could say something like "wow your BF sounds great, he really must trust you getting prettied up and having guys buy drinks for you while he's away?" or simply say "why is it beautiful girls with BFs never seem to have them around when they go out?" This is an art to say the least. You're going to come off as pretentious, but if you've telegraphed your interest so that you're getting a BFD, you may not have much to lose anyway. It's important to use C&F with an air of confidence - you'll get called a Jerk possibly, but use that to your advantage - say "yeah, I've had a lot of girls tell me that". It's really a game of psychological chicken.
Also, consider using the BFD girl as a pivot for a secondary target if possible. If the BFD girl seems a lost cause, you can use her response as a rebound for another girl in the same social setting. Offer "yeah, girls like your friend are ALWAYS talking about their BFs at a club / gathering / wherever you are." to your secondary girl, this will open rapport. You've got to do so with a knowing wink or some other subtlety, but with an air of confidence. Other girls know why women use a BFD, play on this, make her know that you know it as well and you'll develop rapport.