Why didn't she tell me she had a boyfriend? what do I do now?

so I saw this girl last night at a party that I haven't seen in years. In fact, she was my senior homecoming date, but we never really hit it off. Last night we were flirting the whole time, and she made sure I got her number because she wanted to see me at a party this weekend coming up. None of her friends who saw what was going on, or did she, even remotely mention a boyfriend and made it pretty obvious that she was probably single. We texted for about an hour today and everything, then I checked her Facebook page and noticed she had a boyfriend. She finally mentioned in a text a few hours ago that her boyfriend was coming in town for her birthday.

I don't really want to be involved in any form of cheating, but I also don't want to drop her cold. How should I go about dealing with this? keep talking to her but don't make a move? please help!