What if I told you that I broke up with my girlfriend so that I could date her friend?
I would say that you should have more respect for your girlfriend and have more empathy for others.
I just think that their friend is more attractive than my original girlfriend.
I think I'm a hypocrite if I say that I don't get attracted to my partners friends as well, but I think what you did was really immoral and deplorable as if your ex girlfriend finds out she may get really angry and really hurt how about what you did. I would try and speak with her and apologize, and make sure that her feelings are not hurt in the process.
she's fine.
You should have a conversation with her before you take these types of actions so that there is a mutual understanding rather than hurting her feelings.  If you are honest with her she might understand and than she can move on with her life without incurring mental harm.