Well, part of our design is to remember things that hurt us or even caused us to feel bad about ourselves or face rejection. It sounds like your friend has not found a way to resolve what she experienced with this individual she thought was safe but turned out to be toxic. Human beings are designed to navigate and problem solve and part of that includes remembering things that caused us a lot of stress and discomfort. She may have really liked and trusted this person, perhaps more than you did so for her that person turning toxic left her feeling something she has simply not really resolved yet. Hense the saying, "We may not remember exactly what someone said to us, but we NEVER forget how that person made us feel".