Your DP may still want a relationship with his mother out of fear, obligation and guilt but that does not follow that you and your DD have to.
The threat to abandon need not be explicit or conditional ("If you don't do something or if you do it – I will ditch you"). It is sufficient to confront the narcissist, to completely ignore him, to insist on respect for one's boundaries and wishes, or to shout back at him. The narcissist takes these signs of personal autonomy to be harbinger of impending separation and reacts with anxiety.
The narcissist is tamed by the very same weapons that he uses to subjugate others. The spectre of being abandoned looms large over everything else. In the narcissist's mind, every discordant note presages solitude and the resulting confrontation with his self.
The narcissist is a person who is irreparably traumatized by the behaviour of the most important people in his life: his parents, role models, or peers. By being capricious, arbitrary, and sadistically judgmental, they moulded him/her into an adult, who fervently and obsessively tries to recreate the trauma in order to, this time around, resolve it (repetition complex).
Thus, on the one hand, the narcissist feels that his freedom depends upon re-enacting these early experiences. On the other hand, he is terrified by this prospect. Realizing that he is doomed to go through the same traumas over and over again, the narcissist distances himself by using his aggression to alienate, to humiliate and in general, to be emotionally absent.
This behaviour brings about the very consequence that the narcissist so fears - abandonment. But, this way, at least, the narcissist is able to tell himself (and others) that HE was the one who fostered the separation, that it was fully his choice and that he was not surprised. The truth is that, governed by his internal demons, the narcissist has no real choice. The dismal future of his relationships is preordained.
The narcissist is a binary person: the carrot is the stick in his case. If he gets too close to someone emotionally, he fears ultimate and inevitable abandonment. He, thus, distances himself, acts cruelly and brings about the very abandonment that he feared in the first place.
In this paradox lies the key to coping with the narcissist. If, for instance, he is having a rage attack – rage back. This will provoke in him fears of being abandoned and the resulting calm will be so total that it might seem eerie. Narcissists are known for these sudden tectonic shifts in mood and in behaviour.
Mirror the narcissist’s actions and repeat his words. If he threatens – threaten back and credibly try to use the same language and content. If he leaves the house – leave it as well, disappear on him. If he is suspicious – act suspicious. Be critical, denigrating, humiliating, go down to his level – because that's the only way to penetrate his thick defenses. Faced with his mirror image – the narcissist always recoils.
It is not possible to have any sort of relationship with a narcissist and if you cannot deal with her at all it is the self same deal for your both vulnerable and defenceless child. You must never leave your DD alone with his mother, ever!!. I would certainly not encourage any sort of relationship with his mother; she will use your DD as narcissistic supply.
You are right to be wary now because she is really buttering you up to do something really nasty; when a narcissist starts acting "nice" you run swiftly in the opposite direction.
Counselling for your own self may well help you as well going forward; you need to find someone who fits in with your own approach here. You need to also find a counsellor that has no bias about keeping families together despite the presence of mistreatment.