This is a case where she considers the emotional support friend to be a romantic rival, though. There's an issue with her believing she understands Asahi's feelings better than Asahi herself, even if it's presented as true in the text, but being insecure to the tune of feeling second best isn't a good foundation at all. I think she was pretty mature to walk away.
From the perspective of the omniscient audience, I can definitely understand her walking away and feel sympathy for Fuuka, but I also think she was pretty reckless with Asahi's feelings by picking her up and then dumping her in a way that puts all the blame on her. It was practically Asahi's first time. Fuuka wanted it to be perfect, it could have ended as merely mediocre, but instead it became something full of shame and guilt for Asahi.
The more I think about it, the more I think I would be furious at Fuuka if I were Subaru.
I also wonder if maybe part of the problem is that Fuuka was in love with the idea of Asahi, rather than the woman herself.
Bless this series. I've read it so many times already both in digital and print. Even if it's another series that ends with their confession, I'll always cherish it.
Is the ending available in spoilers somewhere? I'm thinking of picking up a physical copy, but I'm worried there's going to be a bad end...