beta
Since it's fiction, and it's not addressed anymore, and I think the story has ended, I'll type down my own conjecture.
I think the reason why the ex girlfriend made it a point to say she was an ex, and also why she had to mention about the cutting hair bit, is because she felt bitter.
If she knew that, and deems that girl to be "not the true love" due to long hair, and knew he grew it due to that, then it's likely that she found that out as well, when he dated her.
I mean, I think anyone would be pretty bitter when a casual question about hair makes you realize that you're not the "true love" the other person is seeking.
She likely didn't want them to be happy and decided to strike the two vital points to hurriedly create a crack between them.
It's likely that the girl confessed, they dated under the pretext of possibility and whatnot (because who knows if this seemingly radiant true love of his childhood would even ever appear) but he never did cut his hair, or remove that bracelet that reminds him of her.
Still...
I think it'd be a little more concrete if we get to see the conversation between them when the girl had left, rather than just me making speculations.
I can only imagine him questioning her, and her getting mad about things.
Even if she did appear again, seeing how much he's changed for the "new girlfriend" is bound to rile her up - but I don't think she'll appear again. I imagine it went a little like this:
"What? I just wanted to introduce myself properly to her. She should know that you're the kind of person to cling to your first love like that while dating someone else. If anything, I'm doing the both of you a favor."
He probably didn't explain too much nor did they linger because he was not only able to catch up to the girl leaving, he also didn't cut his hair until it sprung into his mind when she appeared to linger on it.