I don't think that Ryuuji was a dumbass for trying to set her up with Hibiki.
They got along really well, he had a good job, didn't act like a nut case. Hibiki honestly seemed like a good fit, for her, if she didn't already have someone, and he didn't know about that.
I mean, she did tell him but I get why he didn't believe her at first lol...
But if he had pushed after knowing what Hibiki's done I wouldn't trust his judgement anymore. I think he'll come around, but I'd also be really creeped out if my child came home with a spouse twice her age and the same age as me, a parent. She's an adult yeah, it feels like she's barely left the nest so I can see why it's pretty weird for him.