Roommate Question: 1 moving out, 1 staying

[Nate Hananger]

Property is in California.

Tenant A was living in the 2bed/2bath apartment first. Tenant B was added to the lease under the condition that if either tenant wanted to be removed, they would first be required to find a replacement tenant in order to avoid putting the financial burden on the other roommate. The property management company has a tenant release form that must be signed by all parties before anyone is removed/added to prove the transition was mutually agreed upon. 

Tenant B is now attempting to bypass that condition with little or no effort to find a replacement. They have not vacated the property yet and no move-out date has been mentioned by either tenant. 

Is it possible for Tenant A to make a case with the claim that Tenant B is neglecting their responsibility to find a replacement tenant? If Tenant A has evidence the lease is still active, but Tenant B has no evidence Tenant A agreed to assume 100% of the rent, shouldn't Tenant B remain accountable until a replacement is found?   

If not, what is the best option Tenant A has to defend themselves against Tenant B abandoning their agreement?