Tenant's Guest having issues with another tenant. California

Hi All,

So here is the full story, abbreviated version:

- Tenant A brought girlfriend over to the house, Tenant B brought friend over to the house

- Girlfriend and friend get into physical altercation, both asked to leave for a month to cool off

- Tenant A broke agreement, girlfriend still coming over.  A month goes by, no issues until tonight

- Tenant B says girlfriend threatened to hit her, Tenant B gets police and lawyer involved

As a landlord, my solution was to ask BOTH guests (not the tenant) to never come back to the house again, or I will evict the respective tenant.  Tenant B agreed, Tenant A disagrees and now we are caught up to present day.

In California, what are my obligations as a landlord in resolving this issue?  Both tenants pay rent, never had issues with either tenant.  Only issues with guest towards tenant.  Thank you for your insight!