don't do anything... tell your GF straight that you are not going to choose between her and anyone else.. today it is a friend, tomorrow it can be your parents.. wth! this is your life.. if she is so "lovely" she should understand that even you are her BF (or even a husband) it doesn't mean you are her possession and you have to give up the rest of the entire world..

*********************
“You become responsible forever for what you have tamed”. Antoine de Saint-Exupéry