This turned out longer than I wanted it to be

First off, valid concerns coming from her. I've seen the discussions in the comments. I think the writing makes sense, also for her to think Jae's just vengeful, bratty and bored as he always was. He's been petty in the past, he wanted to hurt people, when he was frustrated or didn't get what he wanted to an extend where he would sacrifice a lot of things, going out of his way and scheming stuff. Sleeping with people he didn't even like, just to one up the guy he despised. Him selling his stuff and moving in with Garam out of emotional turmoil and stubbornness isn't that unrealistic and farfetched considering he is a very emotional and hotheaded person, or at least that's what he's been in the past. His friend hasn't been in the picture for quite some time, it's a shocker seeing this situation unfold before her very eyes, also unexpected to learn Garam was a guy and Jaehyuk's feelings being sincere probably seems super unbelievable in comparison to his behavior and beliefs she's known being his friend for many many years, witnessing his shitty attitude towards relationships, women and homosexuals. I myself would assume this was a prank of some sort and be concerned altogether.
She's not a bitch for calling him out on his past behavior or being distrustful. It's not like he told her every detail of what happened and changed in the meantime. And all she heard was his nagging, complaining and yapping about this ugly shitty person that annoys him and he doesn't even want to date and hopes to get rid of after losing their feelings for them. As in her words.. using him. Nothing evil going on here.

But what I actually wanted to say

1st season Jaehyuk: ewww, homos
Also Jaehyuk: I DidN't kNow yOU WerE so OlD-fasHioneD smh

lmao