The Stooge: The oldest ploy in the world of magic. Plant an assistant in the audience of on-lookers, and allow the audience to believe he/she is one of them once the trick is pulled off.
Notice the misdirection he used: he didn't go straight for his assistant. He first pulled a random girl out of the group, determined she was too short (which makes no sense, because height had nothing to do with the trick), told her to rejoin the group, and picked his assistant. That way, you don't think the stooge is in one the trick-- he was about to do it do some random girl, right? Misdirection.
As for the lady he pulled apart? I'm taking a guess at a legless woman, and anamatronic legs. Possibly remote controlled by someone a few park benches away.
Really, the man could at least try a trick we can't see through.