Definitely don't lie to your boyfriend over something so small. Yes, this is a big deal but the bloke's importance in your life is small. I'd say go and have fun, but let him know that there's no chance and give signs, like not spending much time without your boyfriend, talk a lot about your boyfriend, and don't give a bunch of flirty smiles or wear sexy clothes aruond him alone. Guys are like Pavlov's dogs. Always the same response to such stimulus. I do think it's the womans responsibility to not turn a guy on. Don't mean to offend anyone, I'm just being pragmatic. How would you feel if your boyfriend just met some chick who he says is cool, but really fancies him, and even knowing how much she fancies him he still calls her and sends her texts?