@krimzonhawka
(Erm for the record, it wasn't my advice, Seonhee was the one who asserted that Bomi didn't have to respond to those girls.)
I feel it's irresponsible to say that everything that happened was Seonhee's fault. We all agreed that this was a direct result of those leeching girls. The problem lies in Bomi's personality.
Those girls wouldn't let up even after Bomi subtly hinted that she wasn't comfortable with being the in-between for that person and Seonhee interjects, insults them for being juvenile and that they could talk to him themselves. They then insult her and demanded why Seonhee wasn't being like Bomi who was, in their view, ok with it. Then Seonhee states she doesn't like them trying to make Bomi be their gopher because Seonhee liked Jaemin.
Bomi wasn't going to clear up anything because she's too nice. If Seonhee didn't go all aggressive, Bomi would've been co-erced to try to be matchmaker for Jaemin and that student. Bomi wouldn't say, 'well I like Jaemin' because she probably doesn't quite know her own feelings yet, hence "so soon?" And if she was honest and said "I'm not quite sure if I like him or not, I'm still sorting out my feelings on the matter," that's not something those girls will let slip by and they'll still insist on Bomi's help and maybe even gaslight her into thinking maybe those feelings aren't love or interest.
Seonhee isn't perfect, but she's clearly a dependable friend. She's not trying to hurt Bomi, she's been trying to defend her. She tells Bomi that she didn't have to answer to those girls anymore as she should and if the situation kept going probably she would've asked Bomi to leave with her.
I wouldn't even call it as dramatic as "betrayal". Up to this point we never even saw Bomi ask Seonhee if she liked him. It's very likely Seonhee just found it unnecessary to bring up and as I pointed out before, Seonhee seemed content with the two of them being together.
If it was true betrayal, Seonhee could've just kept her mouth shut and not even confront her friend and spend more time with Jaemin. Instead we see that she spends her time with Bomi, having lunch together, going home together, etc.
I wouldn't necessarily call it hypocritical either because Seonhee is happy with Bomi and Jaemin being happy. Bomi is trying to back off for Seonhee's sake but we see that Bomi's not really happy about the situation despite being all smiles, so it's different in terms of their feelings. Bomi isn't being true to herself whereas Seonhee only kept her interest in Jaemin private.
It's ok to keep information to yourself but it's not good to lie and say I'm okay with this when you're not. And that's pretty much my view of it, we can agree to disagree on that end.