I didn't try this but I hear it works on bars.
And it may be familiar too... I believe you can work it out with Facebook.
Well, here it is: just act like you know her and say something like "OMG *her name* How are you?!?! I have not seen you for awhile! Tod and I really miss you!
Then when she says something like "hmmm lol... who are you?! I think you are confusing me for someone else..."
Then you can act surprise and say you bump into her and thought she was someone else and then say something to continue the convo like, i dunno, "but you look cooler" or maybe "wow your really look the same! where are you from?!" or something.
BTW if she asks how come you were confused with the last name too then say you don't know the last name of this person.
And you may try another one (again didn't try this.. plz don't kill me ) simple as "How youuuuu doing?!

" like Joey from Friends totally funny but little hard to continue from there, I think it should work.