My[19F] best friend [19F] of 14 years, just took back her asshole ex. How should I behave.

Amy and I have been best friends since kindergarten. We're now both at the same university. Last semester she started dating Jared, another student at the university. I didn't know him very well at all, but Amy really liked him so I assumed he was an okay guy.

About 3 months into their relationship he dumped her very harshly. She fell apart; I've never seen her that bad. She was making plans to try to get him back and acting more like a 14 year old than a 19 year old. Eventually she seemed to get over jared, and she told me more stuff about jared that just make him seem like a piece of shit partner. For example:

He's a "recovering" alcoholic who still binge drinks

He has several mental health issues (mainly depression) that he refuses to even attempt to fix as he prefers to blame the world.

He puts little effort into school and is extremely lazy about getting his shit together. Amy is very ambitious.

anyway she kept texting him all summer because she was worried about him. Two days ago Jared said he loved her, which he never said during their relationship and now they're back together. I think this is a mistake and I'm terrified that Amy is going to fall apart if things go bad again. However, I know I cannot change her mind and this is her choice to make. That being said I really hate this guy. How should I behave here?

tl;dr:best friend got back with asshole ex, how can I not be a bitch about it when I'm so frustrated?