I agree that finding your own relationship with her is the best course. There is nothing saying that she is the one. She has to want it too for another thing. Quite often unicorns decide the dynamic isn't for them as much as someone in the first coupling (or *primary couple* as it sometimes is/and/or becomes) does.
Really this isn't the time to jump ahead. You are just meeting. Try and see it as meeting a friend he has just made. That might help ease the anxiety. If she were just going to be a friend you might react differently. If there is a spark there then great, move forward slowly, if not then its not a big deal. Sometimes when one wants something badly then things are forced and there is more possibility for harming others and damaging yourself.
If nothing else you will gain some education on what you both want.
Take a look around here too. Do a search for unicorns or thirds to educate yourself on what its like for other couples and the concerns of unicorns. There is a lot here to read that may help.
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