A friend who is making you an option and not a priority
Sometimes the friends we make first place in our life, come second. That is especially true when it comes to a guy.
Guys are known to have their friends first but girls will dump their friends for a guy. Then when he dumps her she will come crawling back to you.
If you are in that situation think about it. If she gets a boyfriend again does that mean she will leave you flat? If fshe does that is not a friend.
Friends keep their promises, they don't ditch you because they got invited to do something better. Yes, they do spend time with their boyfriends but the still make you feel like you are first place in their life.
There is a saying "Don't make someone a priority who is making you an option." Don't bend over backwards for someone who won't do the same. Talk to this friend and tell them how you feel. If she still does it then forget her. It may be hard but you need someone who is there for you all the time not just when she is single or has nothing better to do.