Roommates: Roommate Moved on Month to Month W/O Notice

My question involves landlord-tenant law in the State of:

My ex girlfriend moved out about 3 months ago without notice to me or the landlord. I want to stay in the house and just have someone else move in and sign a new lease. I am on a month to month lease and the owner has no problem letting me stay. I have lived there for 7 years already without incident. I am willing to sign a 1 year lease with the owner but his is concerned that my ex will sue him if I remove her from the month to month lease. Since she moved out without notice to the owner can she sue him if removed from the lease. The owner will refund her deposit once I sign the new lease and cover her side of the deposit. It was a bad break up so she will not respond to signing anything or talk at all. I just want her off the month to month lease and cover her half of the deposit. Will this be a legal issue for the owner if he removes her and re-signs with just me as long as he refunds her deposit?

I live in southern California.

thanks

Roommates: Roommate Moved on Month to Month W/O Notice