She can't really understand why her girlfriend is betraying that (and her) and it tears her up, so she blurts out whatever without thinking.
It's completely understandable from my point of view why she'd feel betrayed. I'm completely baffled how the other girl can be so utterly dense though. It's made quite obvious that if she kept that shit up she'd be likely to not just destroy their relationship as girlfriends but likely even their friendship itself. She shouldn't have let things get so far that her friend felt that way. (Can't remember names because it's been awhile since I actually read this one. From the name I was actually mixing it up with another one even. That's Midnight Gravity though after checking, not Gravity.)