I managed to catch the trick where he put the card in the girls pocket, although I haven't seen this trick revealed, I can manage a guess at how he did it.
As with most magic, the simplest answer is most often correct. As to how the card got there, it must have been there for all of the trick. Meaning, he used pickpocket skills to put that specific card in her pocket as he was passing by or had one of his assistants do it while she was distracted by the trick. That card of course was a copy, he has another one just like it already in the deck. Now he does a trick all good magicians know how to do. That is forcing a card. It's where you take a specific card that the magician chooses beforehand, and makes it appear as if the chosen person picks it at random. There are many ways of doing this, along with making sure that a card placed back into the deck moves through shuffling exactly where you want it. This is called false shuffling. Now he simply shows her her card again to "show" that it's in the deck. Through sleight of hand, makes the card "disappear", and then has the girl check her back pocket where the same card has been for the past several minutes.
This is at least how I would have done it had I been David Blaine.