Well, I am not sure what to tell you. You can point blank ask them, "Do you realize what you are doing?" This would be in reference to what they are doing to you.... and if you can, tell the person how it makes you feel, using non-judgmental language. if you know anything about their background, family of origin, you might have an inkling about what the original abandonment was. Maybe if you can get the friend to see his/her pattern with you, he or she will see that this is a common thread in their life.
But, really, one needs therapy to work through this.