I am going to send you a few articles that I think are helpful in order to support your daughter, but also empower her and yourself as part of her support system.
Narcissists have an amazing ability to manipulate through their charm and have a way to turn things around on their partners, thus leaving their partners feeling guilty, feeling to blame, and thinking that it is actually on them to make changes to improve the relationship. They really are masters of disguise and seem to always have an ulterior motive in their actions. It feels like an endless cycle of conflict with highs and lows. The relationship can change from elation and romance, to the rock bottom of despair. It is exhausting for those in the relationship, but also as a supportive person. I am a mother also, and I would imagine watching this cycle of abuse feels crippling but also leaves you powerless. You may want to consider what role you are playing. You may have to establish your own hard boundaries with the situation, not with your daughter, but the "situation" so that you do not accidentally become an enabler. Your daughter may also have to recognize her role, if she enables his behavior? Here is a good article that helps one determine if they demonstrate these characteristics: http://psychcentral.com/lib/are-you-an-enabler/
Has your daughter mentioned a desire to leave the relationship? If so, I think that you and her both will like the article I am sending with this aspect of the conversation. https://afternarcissisticabuse.wordpress.com/life/setting-boundaries/
That article lines out the boundaries that are necessary to set in order to escape a cycle or make changes. It is important to have insight about what you are gaining by leaving, rather than getting stuck in what is lost, or perceivably lost.
Here is a brief guide on setting boundaries with a narcissist as reported from a nice little article.
"There are three kinds of boundaries that you have to put in place. The first is a physical boundary. Depending on the relationship you have with a narcissist, there will be a certain degree of physical proximity or intimacy, or the lack of it. There has to be a clear definition or a very lucid boundary that would be physical. You should think of the extent to which you will be intimate or to the degree which you would be distant. Complete distance and extreme intimacy, are both undesirable. There has to be a balance. More importantly, you need to do something different. You cannot build on whatever you have been doing. You need to do the exact opposite and the extent to which you would do that will depend on the type of relationship you have and the extent to which the other person’s narcissism has peaked. Going soft when you have been hard and being distant when you have been too intimate are ideal ways to create physical boundaries.
The second kind of boundary is emotional. You should start to distance yourself emotionally from the narcissist and not allow your emotions to take control of your actions and reactions. If you have been emotionally distant, then you need to vent some emotions. Narcissists react differently to emotions. They often express that they don’t need emotional support when they secretly wish for emotional expressions and support. If the narcissist person you are dealing with craves for emotional support then you should define the boundary or limit to which you would be emotionally involved. If that person doesn’t need emotional support, actually or secretly, then you should not have any emotional investment. Narcissists have a tendency to take emotions on for a ride and that is where your cutting off all emotional connects will be the boundary as it would send a strong message.
The third kind of boundary is psychological. The masterstroke of narcissists is the psychological impact they have on the other person, typically the one who is codependent on the narcissist. Those who are actually narcissistic will feed on the psychological dependence the other person has on them and will thus further fuel their own narcissism. Setting boundaries with a narcissist has to be physical, emotional and psychological. When the first two are well defined, achieving the third is not very difficult but if the first two are not established well then the third will be unachievable. However, if the third is not achieved, then having physical and emotional boundaries will achieve very little. In other words, it is necessary to have some harmony among all the three types of boundaries."
A couple of great articles about leaving a relationship:
I hope that this has been helpful and that you find the answer to be satisfactory. I am available for continued communication. Please provide any feedback that you have and I hope that the answer will return a positive rating from you for the time and effort spent. Communication is open, even after a rating is provided.
I truly wish you the best and please let me know how to further support you..... Thank you! Jules