So my advice is this:
Record it if you can. I know it sounds sneaky but, it has helped me shatter my denial. I have a whiteboard in my living room and I write notes as I can't always remember what my alters are doing when they switch out. So I put a note up asking if some of my alters would record themselves. I sat and watched the ones did and it helped massively and whenever I experience the denial I go back and watch them again. Obviously, don't record it if they are massively distressed
I have over 18* parts. You have to remember when she switches she is no longer "Jodie" but the part who is present. DID is a traumatic disorder and is formed at key development. It is entirely normal to experience denial and often goes hand in hand.
I have an "angry" part in the form of a 13-year-old boy. The thing to bear with angry parts or persecutors is that they are protective despite the behaviour. Mine was formed due to constantly being denied help from the services and people that should have protected me from the traumatic circumstances of my own childhood.
The latest and current theory says that's everyone is born with separate ego states but, trauma can interrupt the development to become integrated and they form barriers between them.
This is not a death sentence for your daughter, many people with DID go on to be successful in life.