Yuu's words about Hikari are, and I'm not surprised, too ambiguous. They fit both with the fact that he was able to talk to her and open up about his relationship with Yami, and with the fact that Hikari is deliberately avoiding him and doesn't want to talk or even see him.
I think Yuu managed to talk to Hikari and tell her everything, which is why he seems so calm about her and is primarily concerned about Yami, who, based on the words of his friends, confirmed that they are worried about her disappearance.
That, or he tried to go look for Hikari at her house and she rejected him and refused to see him, or she simply runs away from home when he shows up. Like Kazusa did with Haruki in WA2.
But we know him well enough to know that he at least made some attempt to talk to Hikari. Or she went looking for Yuu. Hell, they're neighbors. If the guy was able to act like a stalker by going to pick up Yami at her school (yes, WN confirmed he really did it), I definitely see him at least trying to see Hikari.
That's why he seems so calm and confident about her, whether he was able to talk to her or not. If he was able to talk to her, they've already made some kind of agreement to try and save Yami and leave in stand-by the confession, or something like that. Once again, let's not underestimate Hikari's savior complex.
Hikari was precisely the first to defend the theory of "Aya-chan did nothing wrong" in chapter 41.