It's not kidnapping.  If your friend is not married to the children's father, the starting point, under the law, is that your friend as the mother has legal custody of the children, until and unless a court orders otherwise.  So, as long as there's no court order saying otherwise, she has every right to pick up and move, live wherever she wants to live, and to bring her children with her.  She doesn't need his permission, or anyone else's.

She won't be able to get a custody order in Nevada until she's been there for 6 months.  However, a few photographs of the inside of the man's house, dropped off at the police station on the way out of town, might keep him busy long enough so that she could safely wait that long, without him going to court in California first.  Even if he did file for custody in California, I think his chances of winning that would be slim and none.