Well, according to web novel raws, the childhood friend is not out of the game yet. She confirmedly loves MC according to the web novel. The side story you presented was before she finds out about MC's 'death' and she's being tsundere in there. Once she finds out that he is 'dead', she will show her true feelings. And MC finds out about her feelings and how mentally unstable she became after he left in her diary. He cries about leaving her and decides to find her. So now, both of them are looking for each other. The childhood friend decided to confess when she finds MC even though she thinks that he won't choose her. So, nothing can be told about their relationship as of now.