Riko clearly has her part in what happened to Jun in middle school and the like. Whatever her reasoning or rationalisation, she isn't blameless in the events that alienated him from his peers.
but it does suck for her, here and now--not only fighting a losing battle against Saki, but now being effectively lead on by Jun (arguably inadvertently at this point) while he's actively getting more and more involved with Saki (or more accurately, being pulled in & down by Saki), and it hurts seeing how happy Riko was in accepting his confession, and what it'll do to her when she finds out the truth, when she's making this effort to turn herself around and chase her feelings and try and do better by Jun.
But it also makes for more compelling drama, so story-wise I'm all in.
thanks for the TL.