Legal Question in Real Estate Law in California

I think you mean a prescriptive easement. I'm having trouble imagining a scenario where the neighbor would have a prescriptive easement as they must be in continuous use (5years), "open and notorious"(gives you or should give you notice that it's being used), and the use must be "hostile" (without your permission) to you.

To prove a prescriptive easement is a lot more complex than it sounds from the above description. If this is really a serious issue for you, you should pay for a consultation with an attorney.