A 3rd party selfishly inserting herself into your r/s certainly complicates things. Her actions were deliberate. His,.being BPD, were probably more driven by lack of impulse control or his core fear of abandonment, which she may have fed. She's the outlier, and you know her motivations, not much you can do about her.
How much have you learned about BPD? The behaviors are often what we react to, but we can be proactive by understanding and validating their core fears. I thought I did a good job with my former SO, until our r/s went off the rails, and the old methods no longer worked. Maybe this can help for a start:
The Do's and Don'ts for a BP relationshipValidation of his feelings can go a long way. If you communicate, start with validation (
see here for more) and resist the urge to talk too much about your feelings or the situation at first.
T.