That's great! She must really like you.
Her friend from out of town came in. I don't know about the rest of you, but if a friend from out of town visits me, I'll make damn sure to spend time with them... even if it means cancelling previous engagements because I can do all that other stuff some other time.
So if she ain't cancelling on you because her friend from out of town came in, that's a mega-win in my book. And something fun too, like the zoo. My brother always takes his first dates to the zoo... "bitches love animals" he says (short hand explanation, the long explanation goes into more detail, but that's the gist).
So the zoo is not only a great date place, but also great for out of town visitors.
You can impress two people for the price of one. And girls take the opinion of their friends apparently more serious than guys do when it comes to who they date [citation needed, google it yourself].
This is where the 2x1 deal on impressing people starts turning into a real bargain and a possible fast track.
Or... she's bringing her old BF along to make him jelly. Or maybe she's shy/unsure/socially anxious. Or maybe you should try to turn this into a group outing- to test the waters out, get a feeling for the dynamics. She can see what type of friends you have, because the company you keep speaks volumes on your character. And your friend can talk you up.
Not enough details to go by here, stranger.
But from what you do mention, I'm saying the future and it's possibilities are so bright, you better take some shades with ya'.