California law requires owners of fenced-in neighboring properties to split the costs of keeping up the section of fence separating the properties. The law does not apply if one of the two owners does not have fences on the other sides of his land.
A neighbor who later fences in her yard becomes liable for the upkeep of the shared portion as soon as she installs the other fencing. She also owes the neighbor a proportion of the estimated cost of installing the dividing fence section; the amount is based on the current value of the fence.
Talking to the neighbor and mentioning the law if he won't maintain his side of the fence is the first step to solving the problem. A person who cannot get her neighbor to maintain his side of the fence after asking him can either repair the fence herself and sue the neighbor in a California small claims court or contact an attorney for help.