As far as the pimp theory, I doubt that a prostie would give her number to your aunt. Plus, a prostie would have approached you and not ignored you. They don't want to lose business. I'm guessing it was a regular girl.
The fact is, guys move in quickly on lone wolf women. That is exactly what the tall dude did. Was he eyeing her as she walked in the coffee shop? Possibly.
What I don't know for sure is why she ignored you. As discussed, she either wasn't attracted to you and completely ignored you (a bytch move, but plausible), or she GENUINELY did not think that you were you.
Did you tell her what you looked like on the phone? Did you text her to say, "I'm at the coffee shop. I'm right next to the counter; see you in a bit"?
Don't have to answer that. Just food for thought.