I have had family for both my girls and then had a friend for each as well. Not any of the 6 godparents of my two girls would get them if something happened to us. So for me it was purely a gesture to say that they are important people in our lives. I would think about letting your friend be a third godparent, even if only you did promise her. It may not be her pride that is hurt, maybe she was looking forward to it, and genuinly feels let down? oxo
PS saying all that, with DD1 I no longer speak to or have any contact with my friend who is a godparent. I am fine with that, as my sister and brother are still her godparents. It means nothing now that my friend isn't invovled, no big deal. But at the time it was an important gesture to her.
PS my understanding of the godparent is to be a spiritual guide in the child's life if they so choose. I have to say though, in my experience with my own godparents, and my childrens, this isn't really something that is practiced! So for me it is more a symbol of someone special to my children. And I know many people who have no contact with their godparents, just because that's the way it's turned out.