New yapping dog is disturbing duplex neighbor - What do I do?

[Carrie Purkeypile]

I own a duplex in California. I have two good, steady, paying families as tenants. 

Tenant A has been there for many years and has three pets that do not cause problems. 

Tenant B just acquired a dog yesterday, and informed me today. Apparently the dog has been barking since it came home - all night, and all day. Tenant A isn't pleased. I do allow pets (except certain breeds) and charge monthly pet rent. 

I let tenant B know that the dog has already been reported for barking too much and asked if they think the dog will calm down in a few days. I understand that they probably don't have much control over the dog barking. Do I need to set a reasonable time limit for it to adjust and calm down? What would you do?