Property line dispute with neighbor.

[Luis Felix]

Ok here we go, my mom has had her house for 25 years. She resides in California. The fence is and has been on the neighbors property by a couple feet the whole time she has lived there. The lady living there for the last six years just realized this and now she wants to put the fence back where it belongs. This would make my moms addition that the house came with to close to the fence line. Is there anything thing my mom can do to stop this or is she out of luck? 

Thanks in advance for your time.