Seek legal counsel. Civil Code section 1980 et.seq. is a detailed on convoluted section that deals with property that has been "abandoned" There are specific steps that must be taken if abandonment can be established.
As to changing the locks, that is only improper if there is a tenancy involved. Again, seek legal advice for that determination.
A proper response would require a thorough investigation into the history and background of this relationship. The information provided above is just that, information, to be used as you see fit.