....Oh. I just realized. Tsubaki is totally the other childhood friend he protected in that flashback, isn't she? Meek, black-haired girl?
Anyway, I feel like people are overreading her walking away with her toxic bestie. She's worried, but isn't he the one who told her to leave? Who explicitly told her he wanted to be alone?
Her common sense has obviously been quite twisted by her bitch friend, but I don't think it's out of character to think she'd be respectful of his wishes even when he's being self-destructive.