She will. You're too harsh on her, as I doubt you're judging Cindy the same way for being moist for Axel or Ian. At least Alison doesn't go behind anyone's back. Nothing wrong with being a bit of a player/slut when you're single. She won't come crying to Ian about Jeremy because he's ready, willing and able to give her exactly what she wants, and she's using him as a booty call, nothing more. She's not dumb enough to date Jeremy and she's well aware that he's a womanizer. Though if she learns that he has a proper relationship and is, in fact, a cheating bastard, she might call him out and end things out of solidarity - I don't think she would accept the role of "the other girl").
You can't tell what she should or shouldn't have done because we don't know her full story. She's not Ian's girlfriend and doesn't owe to be "faithful" to him and wait, and wait, and wait AGAIN for the moment he finally has the courage to follow his desire and claim her. Keep in mind that she probably spent her entire youth waiting for Ian to stop being stupid; he had all the chances and opportunities in the world to get her if he wanted to but decided to date Gillian instead. She's done waiting or competing with others. Alison just didn't want to be a loser that night and got what was available. If Ian is indecisive and plays hard to get, it's on him, and he has no right to blame her for getting along with Jeremy. Jeremy is the one to blame for being a shitty bro.
And to be fair, being in her shoes, would you not do the same? Let's imagine if Ian was more outgoing and desperate to get laid and Jeremy was out of the picture. And he had 2 options: 1 - I guess Lena, who he fancies but doesn't really know where she stands, and 2 - Alison, who was in his friendzone but is VERY obvious and flirty. So, in this scenario, would you really deny Alison and go for a girl who Ian likes and connects with but uncertain about her motives? I think you'd call Ian an idiot if he passed that opportunity with Alison.
Alison has her faults: she's quite self-centered, and, just like Louise, seems to be needy. But being outgoing and open to any kind of adventures is okay in my book. I just find it a bit of hypocritical that so many people here are salty and slut-shame Alison for basically just having fun. For now, I think Alison is innocent, even if she ends up fucking Jeremy.
On a side note, I think Robert might just be the perfect guy to be Alison's fucktoy. It'd be funny if they hooked up only for Alison to cuck him with Jeremy.