It's challenging to provide a definitive answer without knowing all the specific details and dynamics of the individuals involved. However, I can offer some possible explanations for why your ex, who has borderline personality disorder (BPD), might start dating your best friend and involve her in his family life relatively quickly.
Intense and Impulsive Relationships: People with BPD often experience intense and unstable relationships. They may idealize someone one moment and then quickly shift to feeling devalued or abandoned. This emotional intensity can lead to rapid and impulsive relationship decisions, such as starting a new relationship shortly after a previous one has ended.
Fear of Abandonment: Individuals with BPD often have an intense fear of abandonment, which can lead them to seek new relationships quickly to fill the void left by a previous partner. This fear may drive them to latch onto new connections as a way to cope with feelings of loneliness and abandonment.
Idealization and Devaluation: In BPD, individuals may idealize their partners in the early stages of a relationship, seeing them as perfect and putting them on a pedestal. This idealization can lead them to move quickly in the relationship and introduce their partner to significant aspects of their life, such as their family, as they might believe they've found their "perfect match."
Desire for Intimacy and Connection: Individuals with BPD often yearn for emotional intimacy and deep connections with others. They might feel an intense need to bond with someone quickly and believe that introducing their partner to their family will strengthen the connection.
Impaired Boundaries: People with BPD may struggle with maintaining healthy boundaries in relationships. They might not recognize or respect the boundaries between themselves and others, which can result in overly fast relationship progress.
Emotional Rollercoaster: Relationships involving individuals with BPD can be emotionally intense and turbulent. The new relationship with your best friend might seem exciting and promising initially, even though it might not be sustainable in the long run.
It's important to remember that each individual is unique, and BPD can manifest differently in different people. If you're concerned about your ex-partner or your best friend's well-being in this situation, it might be helpful to encourage open communication between them and support them in seeking professional guidance if needed. A mental health professional can provide insights and strategies to navigate the challenges that may arise in a relationship involving someone with BPD.