"as friends"... b-but they love each other... can't they do the Hiro/Zero-Two speech and decide to choose to make it happen somehow?
I'm really disappointed in her resolve. Do her feelings only amount to so much?
The thing is, context mattered. Now, just a few weeks ago, she saw what would happened if a vampire and a human felt in love with each other. They fucking died. Now, if that's not enough, her own mother, who she knew also loved a human, and even managed to have a kid (herself) and live with this man, is also, to her knowledge, dead and possibly with her husband too after giving birth to her. As far as she know, even if she loves him and they get together, it would only end in tragedy. I could see why she isn't considering to become a pair with him.
And not to mention she is practically immortal. Now sure Kou could also become a vampire, but are we sure is it the absolute correct decision for him? And even if he did, again, like I said up there, love between a human and a vampire has always, from what she knew, ended in tragedy. If they died together, that would actually be a good ending, but IF one of the survived, they would have to carry on living with tremendous guilt and regret for the rest of their lives. And this would means eternity for her, or him, if she reverted back to human and he becomes a vampire. I want everyone to be honest, would you want to live for eternity with the feeling of guilt and regret that you have killed the love of your life?
As such, with what the characters knew, again, what THE CHARACTERS knew in-universe, not us readers, hell even we didn't actually sure if she would live or not, that's how complicated the situation are, it's understandable why she would dismissed her own feeling like that if all she knew is that they would only ended in tragedy. They might not be together this way, but they can still meet and talk for years instead of becoming one with the sand on the beach.