P has created a relationship that will consistently harm his existing relationship. We all have a responsibility to maintain the relationships that we create and he has gone against that by choosing to be with someone who requires monogamy despite already having a partner who considers their relationship to be primary. But how can it be, really? Do you really think he considers the relationship you have with him to be primary when he obviously is willing to risk it to build a relationship with someone who absolutely does not want it to exist?
In this situation, I would be strongly questioning why my partner is building relationships with people who identify as monogamous and obviously are not happy in the absence of monogamy, I'd be asking why he is willing to treat me badly in order to appease her and why we are not treated equally.
To me, a decent partner who was worth my time wouldn't want the headache that comes with trying to convert a monogamous person to someone who can embrace their partner being polyamorous. They just wouldn't risk harming our relationship in that fashion. I'd be having serious talks with my partner about our future together; he has the responsibility to maintain our relationship, not her, I can't blame her for what she needs from a relationship, I can blame him for trying to give it to her though, especially when he knows that giving it to her would be dismissing our relationship.