[landlord US-CA] is it tenants or landlord responsibility to evict tenants roommate?

basically the tenant has a roommate that’s not on the lease and they are constantly arguing and the tenant told the roommate to leave but she is refusing and now he’s asking me for help to kick her out. I believe it is his responsibility to kick her out seeing that his name is on the rental lease and she is technically his guest. Not sure what to do in this situation but I could be wrong. I’m still getting the monthly rent money on time and it’s not affecting the rental property or me in anyway. Thanks in advance.