I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. I have been a CA licensed attorney for more than 3 decades.
Under the Los Angeles Rent Stabilization Act (RSO) you may raise the rent of your tenant by 10% if you have actual or constructive knowledge that another person not listed on the lease is living there. Actual knowledge would be being told they moved in and constructive knowledge would be you seeing that they had moved in. The amount of time the boyfriend has been there and the frequency you have noted is sufficient for you to know the new tenant has moved in.
The one caveat that you need to be aware of is that the law states that after you gain the knowledge that an additional tenant has moved in, you MUST raise the rent of the primary tenant by up to 10% no later than 60 days after you learn of the move-in. If you wait longer than 60 days then you are deemed to have waived your right to increase the rent.
In order to be eligible to evict, first your lease must prohibit new tenants joining your primary tenant. If that then happens you must serve a written 3-day Notice to Cure or Quit the violation giving tenant 3 days to make the boyfriend leave. Only if the boyfriend is not out after that 3 day period would you then be allowed to file an Unlawful Detainer action against the Tenant and the boyfriend.
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I wish you and yours well in the New Year,