Some girls want a super kind dude, that stops to be kind to everyone, as soon as they date him. That's like to chase after and get into a relationship with a slutty girl and being surprised, that she might turn out to be a ...hurrdurr... slut. Or lets turn that down and say that girl wouldn't sleep around with anyone (like a normal person does (A person with ideals and faults.)) except she is in a relationship with that one person. But she did hang out with the guys before and she won't stop it now, BF jelly or not.
Being kind to people is what defines these people and going against it actually hurt them and make them feel miserable on the inside. Yet their spouses, stemming from crushes, still get jealous. If you get into a relationship with a person, that is seriously kind with people (overcompensating something or not), you better have to accept that side.
In kouhais case I do believe she is hurt on one side (because he is chasing after the other girl), on the other side she is seriously concerned, that somebody (not her, then again her, because she might be projecting herself into a relationship with him) will get hurt. She does see, that that whole situation is unstable AF and she is right. Somebody will get hurt. Even a few somebodies. ... But damn...to tell the "Good Guy" you are into, to stop being nice to others before you are in a relationship (that might never actually happen) with him ... that's forward... I know, that's her way to show kindness in this sitation, but...XD