Unless it's a mistranslation, no, everyone isn't "at fault". Hikari is completely innocent here.
She is still the girl who friendzoned Yuu in first place and did nothing when Yami was slapping Yuu. If Yami is the bad girl, Hikari is the theoretical best friend of Yuu who didn´t even try to protect him.
All this disgrace happened because Hikari has even less spine or convictions than Yuu. Even when she confronts Yami, she doesn´t say practically nothing, Yami is who says practically everything for the guilt she feels about how she betrayed Hikari.
But, of course, she is a girl, she has right to being a passive weak girl from manga shoujo who stays paralyzed while other girl kiss her theoretical love interest, and runs away from Yuu, abandoning him in the gym, instead claim responses from him.
This girl is a coward and running away just after scream "I hate liars" is a pathetic act of cowardice but even the worst Yuu never did. Yuu always claimed responses from Ayami, always presurred her to take seriously the relationship.
Hikari is a complacent pimp who forgave Yami after Yami tried to whoring her into an old dirty man. See? Another thing Hikari did wrong.
And this is the girl, also, who even in the confrontation with Yami, is still in mode "Aya-chan, you are a good girl, you still loves Yuu, you are not a bad person, you are a good girl who did only a thing wrong".