Legal Question in Real Estate Law in California

That is one possible outcome. However, there are two other possibilities: (1) Neither is liable. (2) Only one is liable.

Start out with the notion that liability for accidents is based upon negligence (with an exception -- there is also automatic liability that is imposed upon parties that engage in extrahazardous activities such as using explosives or keeping wild animals). In the case of a falling tree limb, the injured party must establish, among other things, that the owner (or owners) of the tree were negligent. Some accidents happen without negligence. If a tree in apparent good health unexpectably and unforseeably loses a limb that injures someone, the injured party might not be able to prove negligence. So, possibly, neither property owner is liable.

The next issue, whether one or both is liable, probably depends upon ownership of the tree. The ownership of a tree usually determines who is responsible for keeping it safe and hence who is responsible if an injury results from faulty maintenance by (for example) failure to trim off dying limbs. The Civil Code says that a tree belongs to whomever owns land its trunk stands upon. If the trunk is on the boundary line, it is co-owned by both neighbors. In this case, both would be responsible for maintenance and both liable for an accident (usually, at least).