Not a Californian (and I do see that you have a reply that sounds knowledgeable from someone who is), but the neighbor would need to take it to court to perfect his easement--until a court has granted him an easement, it is merely a claim. Your friend should talk to first his title company (who should have discovered that there was a prescriptive easement) and then possibly a lawyer (although the title company should defend his title as being free and clear--that is what title insirance is for!). There are any number of things that can prevent the granting of a prescriptive easement or claim of adverse possession. One of the simplest is for the titled owner to give permission for the use of his or her property.
From wikipedia
http://en.wikipedia.org/wiki/Prescriptive_easement#Easement_by_prescription
"prescriptive easements are hostile (i.e., without the consent of the true property owner). Prescriptive easements do not convey the title to the property in question, only the right to utilize the property for a particular purpose. They often require less strict requirements of proof than fee simple adverse possession.
Once they become legally binding, easements by prescription hold the same legal weight as written or implied easements. Before they become binding, they hold no legal weight and are broken if the true property owner acts to defend his ownership rights. Easement by prescription is typically found in legal systems based on common law, although other legal systems may also allow easement by prescription.
Laws and regulations vary among local and national governments, but some traits are common to most prescription laws. Generally, the use must be open (i.e. obvious to anyone), actual, continuous (i.e., uninterrupted for the entire required time period), and adverse to the rights of the true property owner. The use also generally must be hostile and notorious (i.e., known to others)."
and
"Generally, if the true property owner acts to defend his property rights at any time during the required time period the hostile use will end, claims on adverse possession rights are voided, and the continuous use time period resets to zero.
In some jurisdictions, if the use is not hostile but given actual or implied consent by the legal property owner, the prescriptive easement may become a regular or implied easement rather than a prescriptive easement and immediately becomes binding. In other jurisdictions, such permission immediately converts the easement into a terminable license, or restarts the time for obtaining a prescriptive easement.
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Several months ago we had a somewhat similar situation arrise in our HOA, and I did some digging into adverse possession and prescriptive easement laws. I don't recall all the details, but I do remember that there were several variations on resetting the clock on uninterrupted use, and in some states the sale of a property without the easement or adverse possession being perfected in court resets it to 0. Posting No Trespassing signs and shooing the birds away, or sending your dog to chase them off are good starters, but an appointment with the title company as soon as possible is imperative.