Oh, man, that's gotta hurt!
She's just confessed to him, big time, and he has absolutely no clue that she's done so.
Talk about oblivious!
This, of course, is the hurdle that childhood friends have to surmount.
That the other party has them so compartmentalized in their mind as "friend" that "significant other" just doesn't come up as a concept.
Wake up and smell the coffee, already!
*sigh*
I'm sure he will, eventually...
The mangaka isn't so cruel that he won't.