I can respect this. However, just from my point of view, this sounds like caring more about self-righteousness than about the friendship itself. Worst case scenario, your friend and you both lose a friend. Best case scenario? Nothing happens.
If my friend is dumping me for invading their privacy than they think invading their privacy is friendship ending, which means I had already nuked the friendship by invading their privacy. At this point I'm just making sure my friend is giving informed consent to our friendship. To me "protecting the friendship" sounds like a self-righteous justification for not having to face the consequences of my mistake.
But also something already happened - the initial violation of the friends privacy that you're banking on never coming out. That there's no chance you'll ever use that information in a fight or casually let slip w/o thinking. Which right, the alternative to not telling the friend is that you have to be on your guard about this information, which maybe you see as punishment enough but generally that sorta stuff leaks.
It has in the manga - Hikari has flagged Yuu's evasiveness a bit in the past & this is her "I'm done" moment.
Let's say for them it's very important, for you - not that crucial, but you're not changing your opinion on the matter just because of that. Would that still lead to a confrontation even though you can simply avoid mentioning the sensitive topic and continue to be friends?
So I'll concede that there's nuance here but I think it primarily depends on who I'm talking to. My best friend yes b/c the trust is there to discuss these things, friend who I maybe see twice a year yeah I'll answer honestly but maybe not proactively raise the topic.
Which right, that's a core issue in the manga - Aya trusts her friends enough to tell them all about her ex, but Hikari would see it as Yuu not trusting her enough to tell her about his. Since Hikari has always thought that their friendship is super open and honest, this changes what she thinks about the very nature of their relationship. Which going back to the Q is why yes I have these convos with my best friend.
So when Hikari went from friend zoning Yuu for years to having romantic feelings for him, your position is that Hikari was unfair for not sharing that, right?
Sure, why not, if you think the not confessing is equivalent have at it.
On top of that though, Yuu has also been hiding Aya, is worried about something going down at the festival but doesn't give Hikari a heads up, and then lies about why he went missing, probably b/c he's scared of Hikari's reaction to the info given he's in love with her. So he's also a liar and coward for trying to deflect Hikari's anger/frustration by confessing.