Kym, there are lots of things that lead to developing an eating disorder, which makes sense because the disorder functions as a major coping strategy.
There has been extensive writing about connections between childhood sexual abuse and eating disorder development. I don't know of anything specifically about narcissistic mothers and the later development of an ED in their child (but that doesn't mean there isn't anything out there- there are so many studies that I don't get a chance to see all of them).
I can tell you a few things about being in relationship with someone who has narcissistic tendencies, which might sound pretty familiar to you.
You guys know how Narcissism got its name, right? That dude way back in the day- Narcissus- was hanging out at a lake. Apparently, he looked down into the lake and saw his reflection; and he thought he was ALL THAT. He thought he was so appealing that he stared and stared, and became so obsessed with himself that he eventually fell in and drown. (or something like that...but you guys get the idea!)
The primary characteristic of, and world view of, someone who tends toward narcissism is self-centeredness. Depending on how much of this characteristic the person has is the extent to which they are involved or not not involved (connected) with the world.
It's very difficult to be in a truly connected relationship with someone who is narcissistic. This is because they effectively live in a bubble. If you imagine yourself inside a bubble, anywhere you look, you see your own reflection. No outside information or feedback enters the bubble. All you see is yourself, so all you believe exists is you. You learn to believe that everything in the world is an extension of you.
Anyone that tries to be in relationship with you encounters the edge of the bubble. They simply can't get through. The bubble won't let them pass. They end up very lonely and demoralized- all they are trying to do is reach you, and nothing they try works.
And to make matters even worse, you (as the person in the bubble) experience them, if you even notice them, as part of yourself (since all you can see is yourself reflected in the bubble).
This dynamic is difficult enough when we try to be friends with someone who's stuck in a bubble. But when we're a child and it's a parent that's stuck in a bubble, it's much,much worse.
As children, what we most need is a true, stable connection with our parent/s. If a parent is stuck in a bubble, they can't connect. Our response is to try and try and try to connect, only to continuously be unable to. And, then, we often end up blaming ourselves for our "failure" to make the connection, and we think we are "bad" and "worthless" and not deserving of having connection.
So, you can see how an eating disorder might develop as a way to cope, and as a way to have a "connection" with something (a consolation prize for sure, but at least the eating disorder provides an illusion of connection, right? And that's better than nothing- we are social animals, remember- we don't do well at all without connection).
In recovery from the eating disorder, the person gets to learn that he/she can have true connection in the world- that there are lots and lots of people who do not live in bubbles, and therefore are able (and very much willing) to connect. And over time, as she builds these true connections, the pseudo-connection with the eating disorder recedes. It is replaced by truly fulfilling connections- the kind she wanted all along.