I'm not sure what your specific question is, but all attorneys would advise you to properly give a three (3) day notice to pay rent or quit. The three (3) day notice should contain all of the amount that is due. If the rent is not paid, you will have to bring an unlawful detainer action (eviction) and seek the past due rents as damages. Because of the intervening agreement, I do suggest you speak to an attorney before commencing the eviction process, as there may be something in the agreement that would affect the case.
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The eviction process is a bit complicated and requires strict compliance with the applicable legal rules. So, before you proceed, you should retain a lawyer to assist you and review the agreement you signed with the tenant's girlfriend to see how it may affect your position.
Generally speaking, you would have the right to serve the non-paying tenant with a 3-day notice to pay rent or quit. That notice should state the correct amount due, etc., and comply with the law and any residential rent control rules applicable in your area. After the notice expires, and if the tenant still fails to pay the due rent, you would be entitled to file an unlawful detainer lawsuit to evict the tenant.
But you should not proceed without first getting legal advice from a lawyer about the terms of your lease, any specific type of eviction notice required by the lease, the impact of the "agreement" and the legal requirements involved in the process.
The information presented here is general in nature and is not intended to be, nor should it be construed as, legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.