Cash for keys in Los Angeles rent control unit

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Right now I'm working with an out of state landlord who wants to retire. 

He has a Duplex (front house and back house) that both have tenants in them. The Duplex apparently falls under the RSO. He doesn't know what to do because he never registered the rent with the HDICLA and there is no written lease agreement. The tenants basically have just been living there for anywhere from 500-700 dollars a month depending on what they can afford. 

Now he wants to sell the house to an investor but he needs to get it vacant. 

What are his options? 

I told him about a Cash for keys offer but he's tried to give the tenants money to leave before with no success. The tenants in the back house were/are willing to leave but the tenants in the front house aren't budging.

I'll also add that the tenants in the front house are his relatives (his sister and sisters family etc). From the what I can gather, the owner doesn't have a strong enough relationship with his sister (front house tenant) to play the "family card" or appeal to the humanity in her. 

What are his options? He lives in Colorado with his family and needs to get rid of the house in Los Angeles. He has a buyer but the buyer wants the house vacant. 

What are his options? There's no written lease, never registered with the HDICLA, i'm actually quite lost in this situation.