Legal Question in Real Estate Law in California

You are responsible......the overhanging tree limbs are a trespass, and as their owner, you are the trespasser. The trees might also be a private nuisance.

Unlike most trespasses to land, the victim of the trespass need not go to court for a remedy; instead, he may elect to do the pruning himself, back to the property line, but if he so elects, he cannot recover the cost of the pruning from the tree owner. Further, the victim must do the pruning in a "non-negligent manner" according to one appellate court decision; this is usually taken to mean "in a way so that the tree isn't killed or become a windstorm blow-down hazard." Invasive tree limbs and roots can also be, or become, a private nuisance, giving the victim a choice of theories upon which to go to court and request money damages, an injunction, etc. if he'd rather not go the self-help route.

My advice would be that before either of you acts unilaterally by trimming, suing or whatever, that you talk it over, hire an arborist to give you some ideas on what is safe and healthy, and develop a plan that keeps you out of court, keeps the trees healthy, acknowledges your responsibility, and above all restores some friendship and neighborliness.