I don't know about you guys, but for me, birthdays are the most sensitive days. What they say is true: if you don't cry at least once, it's not your birthday. I remember once I cried only because I THOUGHT nobody was going to come (I think only two people texted me saying they couldn't make it).
In the end, most of my friends came to my house, but I can only imagine how much I would've cried if nobody actually showed up. That's what happened to the girl in this story, and hey, I don't blame her for wanting to cut everybody off and start fresh; I would do the same thing.
Sometimes, it takes a big disappointment to see things more clearly. Maybe you think some people are your friends when they're not, and that's normal; it can happen. When you're very lonely, you tend to overestimate some people's presence in your life and fail to realize they might have other, closer friends.
It has happened to me, and though it's sad, it's definitely not the end of the world. It isn't anybody's fault either. In friendship, much like in romance, people don't choose who to be closest to based on an objective list of characteristics; they choose based on timing and pure vibes, and there's no point in trying to match the vibe of someone who doesn't naturally vibe with you. You have to pick up your stuff, go out, and find someone who naturally matches your freak, and trust me, they exist.
I might be overstepping by assuming the birthday girl has to find other friends, but I'm just trying to empathise with her and follow her intuition. I'm sure this isn't the first time she's thought about this. This must've been the drop that made the glass overflow.