A) Let's not pretend that a heterosexual man and a heterosexual woman living together/ living in the same house, whatever, is the same as two heterosexual woman living together. It's not, and making a wishy-washy excuse like, "You should trust me, a good relationship is based on trust", blah blah blah, should never be tolerated in cases like this. I mean, come on.
B) On top of that, she didn't tell her boyfriend about this, that's pure Deception. No excuse for that. And isn't Ko-se-Gyeong her ex boyfriend or something,?? I forgot.
Maybe not, maybe he is, if he is, that's extremely worse, if not, that doesn't make the case any better than it is right now. I hope I don't have to explain why it's way way worse to live in a house with your ex boyfriend when you are in a relationship with a new guy.
Now, let's say this Ko-Se-gyeong guy is actually Gay, and she thought it's okay to live with him since there is no chance of a sexual tension . Still, not telling his boyfriend, for whatever reason, is absolutely a bad taste choice to make.
C) Relationships are based on trust, yes. However, one must recognize one simple aspect of human relationship. "If you don't give reasons for someone to trust you, more importantly, if you give someone reasons to doubt you through your actions/inactions, you don't deserve to be trusted to begin with",
"Also, trust isn't a one time thing, like, today I trust you, so I should trust you in the future at any case, Trust is something that has to be maintained regularly through trustable actions, regularly." If you don't think so, then please, be in a relationship with a Martian, human minds don't work that way.
D) Relationships are also about giving up some freedom. That's how relationship works. I may think it's okay for me to go to a walk every now and then with my ex girlfriend and go to a drive or whatever, and it's part of my freedom of choice or whatever, but my current girlfriend may not like that, rightfully so. Now, whether I like that or not, it's obvious that if I don't relinquish that part of freedom, I will lose the relationship. It's a matter of choice, which one is more important to me. That's why I don't go and spent nights and my lady friend's house when I am already in a relationship with someone, and neither does my girlfriend with any of her guy friends. That's not just common sense, that's basic human decency. If you can't do that, mainstream relationships aren't for you, maybe be in a open relationship or whatever.
In this case, she didn't even tell him to even verify if he is even okay with that or not, that's just bad decision after bad decision.
Conclusion: She has obviously done the wrong thing, obviously doing the wrong thing, Period.