Ugh - so sorry. He put the sign out on the lawn before telling the child? She's likely to hear about it from friends before he talks to her, which is wrong wrong wrong. But he is who he is, and there's no easy way to tell her. All you can do is reassure her that her dad loves her. If he can find time to tell you what words he used to tell her, that will help - you can try to be on the same page.
I'd de-emphasize the girlfriend and make it more about his job, whether it's true or not. Say that he is watching out for her by going for the best job opportunity, that you know it's hard for her to see that but these are things that grownups have to decide and kids don't need to have it all figured out.
Stress to her that she has managed the change from 2 parents living together to 2 parents living apart, and make sure to reassure her that so many other things in her life will stay the same. Some kids can understand that they get a new teacher and a new classroom every year, or go to a new camp or summer program - I know it's not the same as "your dad's house is getting a new owner" and "your dad will be living farther away", believe me. But some kids benefit from looking at changes that ARE manageable, and making the next step to changes that seem unmanageable.
If she's sad, angry, scared or disappointed, listen to her and let her have those feelings. Don't whitewash them. Focus more on what she thinks and not on what you think. It's hard to keep your own resentment and anger at how he's handled this - but it sounds like he's not a very responsible co-parent in terms of connecting with you so you can both address this together. It may be that this is the first of quite a few disappointments she will have, and an example of other parenting decisions he will make. I don't know. But if it is, you can't protect her from all of them, or maybe any of them.
Talk to your children's librarian at your town library or a larger library that is part of your library's lending system. There may be some excellent books that deal with this. Parents leave for a long time for a variety of reasons - career moves, military deployments, job transfers, and so on. Perhaps seeing herself as one of many kids will help her rather than depress her. The main thing is to reassure her that her dad loves her and always will. Start working on things like Skype or FaceTime so she can talk to her dad face to face.
I would NOT make too big a deal about all the great things California has to offer until you know for sure that she will be visiting him there. The logistics have to be worked out and it would be unfortunate if she were assuaged with thoughts of Disneyland, Hollywood or some other big landmark/attraction. At this point you don't even know if his move will "stick" or his relationship will work out. If it is a permanent thing, your daughter will have time to meet this woman and ideally not blame her for taking Daddy away. If it doesn't work out, the less your daughter fixates on it, the better.
Good luck - I'm sure this is so hard for both of you.