It's not the same exactly and it has nothing to do with Emma being a close friend. She mirrors Ian's actions, but she never starts with it.
1) First time she goes for Jeremy because she thinks that he's her best chance at getting laid and because Ian ignores her or beats around the bush. If he makes his intentions known, she's swooned.
2) Second time she sees Ian kissing Emma while she had plans to spend the night with him. Naturally, it makes her angry, so she acts out and grabs Jeremy along because 2 can play this game. It's not because of the "close friend" thing. It's because Ian knew she was willing to go with him that night, and he chose to hook up Emma instead. In Alison's book, it's rejection. It's a huge blow to a girl's self-esteem that she restores by hooking up with Jeremy. Now, if Alison couldn't come to the club and he hooked up with Emma and later confessed to Alison about it, then she would be catty about it but she wouldn't reject Ian's advances because she likes being chosen among other options " You couldn't live with your own failure after fucking those other girls. Where did that bring you? Back to me ". That's why she agrees to hook up with Ian if he already hooked up with Cherry. It makes her feel like a winner "He sampled the merchandise but chose the best". If, however, Alison will get to meet Lena, and Ian is present, she'd expect him to choose her to spend the night with. Otherwise, she'd react the same way she did with Emma. "You showed your priorities". Her attitude is basically like this: "You can play around without me present, but when I'm in the same room with you, remember that I'm your Alpha Bitch, or else I'm looking for someone who will appreciate me". She can handle Ian playing around with some girls on the side but she won't stand for the humiliation that comes with rejection.