My question involves personal property located in the State of: California

My Mom Has an eccentric/control freak neighbor who is home all day with 2 full time workers working on his yard/home since he moved in 10 years ago. He has an eco garden that he's done himself and he has his workers do odd things like dust his palm tree's.

Today he pruned 1 large maple tree of my mothers w/o consulting her. I asked him to stop until he consulted with her, he agreed and kept his workers pruning the tree's and had to be told a second time.

Because he's home all day, I'm sure he'll just wait for everyone to leave before he sends his workers over to do who knows what.

What are my mothers rights? She's an incredible docile women who shy's from confrontation. I say write him a letter asking him to keep off her property, sent certified. There after taking pictures of his antics. He's a strange man and he seems to not understand property bounderies.