1) The good and ideal way.
Have a proper break with your gf first, tell her that you have
grown tired of the relationship and wish to have a proper break.
Once she agree then go for her friend.
2) The safe way
Call your gf friends and ask her out ask a friend, know her,
understand her as a friend and see that if she is suitable for you.
You may find her attractive at first but when you know her friend
better then you may realise that she may not suit you.
3) The mean way
2 time your gf by going out with her friend. But before you used
this method you need to be cunning, lots of free time, lots of cash
and handsome enough. Or else you will fail so badly that you end up
losing the 2 girls.
By my experience I can predict what would happen
You will find your gf's friend to be much more attractive than
current your gf cos you are sick and tired of her. If she can stand
your picking on her, the relationship will go on but if she cannot
stand she will break up with you.
But be warn if you do not break up properly she may bad mouth about
you in front of her friend and you still end up losing both.