your answer is in your own post. the girl has already shown and admitted that she is only friends when it is convenient for her. this sounds like a one way friendship. what would you have to gain by being friends with her. is she kind? giving? does she provide you with laughter and good times? as long as you are going to gain as well as receive from the relationship, then the relationship can be of benefit. if there is no mutual exchange, then you have to think about reconsidering. either case, you must keep in the back of your mind her nature of abandonment when she gets a boyfriend. you will likely get left by the wayside again should another man come in her life, so be prepared for that.