Hmm, so it does turn out to be the case that Aiko is mutual friends with both of them. Last chapter, when I saw that his female friend had the same hairstyle as the girl in the flashback sequence to Miyuki's first day of school, I wondered if that was the same person (Aiko). Looking back now, Miyuki telling Hiroshi that she's used to staying out late after school should've hinted towards her being somewhat of a delinquent, hanging out with Aiko, dressed up and behaving quite differently from when she's in front of Hiroshi (not as prim, proper, or well-mannered as she usually makes herself in front of him).
Also, speaking about that flashback sequence, she's not fooling anyone other than Aiko about looking down on love as a "stupid idol to worship." We all saw how flustered she got when he blushed at her visage on that first day, there was nothing platonic about that; she wouldn't have endeavored to change and hide her real personality from him otherwise.