Not only you refuse to think about the situation from Yami's standpoint, you don't even want to think about it from Hikari's standpoint and consider her personality. Yami must do something as a friend and Hikari may do whatever she wants. Now this is a logic that makes absolute sense.
But I can answer for you. As Yami's friend, Hikari should ask Yuu if he still likes Yami. And if he does, she should help them get back together. Because they are friends, right? That's what you want, don't you?
I commend you for at least trying to put yourself in Hikari's shoes (again). But my answer again would be "that's what Yami might have wanted to avoid".
Just tell them the truth. It feels right and fair to everyone, doesn't it? Because everyone likes truth and honesty. At least until it concerns themselves and their relationships.
No one starts a relationship with giving your partner a list of people you previously were in relationship with, who you confessed to, who you had sex with and how many times. No one says "honey, to be honest, when we just met, I liked your friend better because she had a nice ass, but then I ended up falling in love with you because she was already taken". Ask yourself, how many times in your life you lied to your friends or parents with both their and your own interest in mind. And then go ahead, continue to blame fictional characters for omitting the truth.