I'll say it again: there's nothing remotely controversial in this manga.
And to get it out of the way, I'd have zero qualms with a gender-reversed situation. Why? Because there was nothing wrong with the marriage of Oona O'Neill and Charles Chaplin. He was 54 and she was 18. They eloped to Mexico to get married, stayed together to the end, by all indications loved each other tenderly, raised a happy family of eight children (Chaplin was last a father at 73) and are buried next to each other. And since there was the threat of a stern father mentioned in this chapter: Oona's dad, Nobel Prize laureate playwright Eugene O'Neill cut ties with, disinherited and never saw her again. Doesn't seem like she gave a shit about it, and he was the one who died a bitter recluse.
Also: there's nothing wrong with lust. So long as there's mutual consent, sex is ultimately a private decision of what to do with one's own body and urges. There are tons of people who engage in casual sex. The friend here is a Victorian prude who thinks there's a standard way to begin a relationship. Getting down and dirty first and then trying out for feelings is as legitimate as the other way round. Kon'no is clearly lustful for her (one might argue that physical attraction was what got him going in the beginning), so why's everyone dunking on her for being turned on by the idea of getting laid by a hot young man?
The question is rhetorical and I'll answer: because as the audience we know Kon'no feels more than lust for her, and consider it unfair of Hinata to not be honest about how she's feeling, which might lead him to believe her emotions are reciprocal. Her friend's only good piece of advice (other than cautioning her that last-minute backpedalling on a sexually aroused teen can be dangerous if she doesn't know who she's dealing with, which is frankly not particularly brilliant counselling but par for the course) is getting Hinata to put in words how she feels about Kon'no. And the friend drew a completely wrong conclusion from her reply, and was nothing short of rude about it, guilt-tripping Hinata like that.
And that's the only charge I'll grant: she could have told him in advance she felt physically attracted, but at that point she didn't see him under a romantic light. That's a fair cry from being "messed up", it's a common communication mistake, something she could even admit after having sex with him and then the two could work things out. Starting couples do that all the time, it can lead to break-ups and second attempts, it's normal. It would be a mistake regardless of the age gap, and not something to throw rotten eggs at her for. Love is messy.
Sheesh, it's sad times when such a bland story gets everyone freaking out like this.