If it were me, I'd probably go with her. She was going to leave but now not only is she staying local, but she's found an opportunity for you to go with her. To me that sounds as though she (maybe without admitting it to herself) really wants to know if this relationship is worth changing her "probably" moving to california plans for.
I wouldn't go with the hope that you can convince her to stay, but you don't know what you could learn in the next few months. If it feels right and you dont think you'll be worrying the whole time, I think you should go for it.
Good luck man :)