She thinks she sees Amanda Armstrong one day, passing by, but before she has a chance to say hello, or to smile, or to say something (what would she say?) she has disappeared. It mightn't have been her, anyway.
Betty can forgive herself for many things, or, rather, she will argue that many of things that others seem to feel she should feel guilty about are actions she will defend, but not Amanda Armstrong. If she was subversive than Giselle certainly is, and Giselle –
Giselle is very dear to her. That is how Betty phrases it. She can't bring herself to say I love you, even though she has said it to her friends before. She knows saying it now would mean more.
Amanda Armstrong loved a woman, and Betty knows that this, this blatant disregard for society and its rules, in which women must marry, and Amanda's strange compromise for not being able to do this, is why she attacked. Why she had to, for the good of the college, for the good of her generation.
Now Betty thinks that perhaps it was not a compromise, not some desperate attempt to emulate married life, but merely love, plain and simple. And she knows that marriage is not the dream she always envisioned it as.
In this apartment she shares with Giselle, she has found something to hold on to. It strikes her, as she walks in the door, that she would have something to say to Amanda.
And, I understand now.