I dealt with the friend that cant let go when I started dating the person who would become my wife. The main advantage I had was I caught on to what the "friend" was trying to do before she was able to destroy our relationship by doing what your GFs friend did.
Women initially will ignore the friend but over time they can wear them down and when things are not new anymore they start to listen to them.
This friend is usually the "needy" one and their only reason they have for not liking you is selfishness.
In my case I preempted it by pointing out instances where I saw my GF (at the time) was being manipulated. So when the "friend" started to do some of the things to tear us apart it actually worked in my favor because my GF recognized it and it made her distance herself from that friend.
psn- tokila
add me, the more the merrier.
