Removing or Changing Name on Lease Without Consent

My question involves landlord-tenant law in the State of: California

My friend had lived in an apt for some time when he decided to move. His two adult children continued to live there although the lease was still in my friend's name. His circumstances have changed and he wants to move back, but was told by his daughter (and confirmed by the landlord) that a new lease was signed by his daughter and son and they do not want him to move back in.

Is it legal to do this without his knowledge or consent? What are his rights?

Your friend had the right to continue to live in the apartment through the end of his lease term, continuing to pay rent and uphold his end of the lease agreement. He chose not to, thus failing to meet his contractual obligation.

He should be thankful that the landlord simply allowed the other tenants to sign a new lease, rather than take him to court for breach of contract.