First, I am not sure if there is an anti-smoking ordinance in your city. You might want to check that out. Generally, a person is legally allowed to smoke in their backyard - cigarettes at least. Unless there is an ordinance that prohibits smoking (which many cities now have) then the neighbors should not be able to win a nuisance lawsuit for that.
Next, the landlord is legally allowed to inspect the premises. Just give advance notice and try to set up an appointment with the tenant. In cases of emergency, the notice requirement can be ignored.
If the property is being damaged by the tenant, that could be grounds for terminating the lease.
If you go to the link below, you will find an informative guide that explains the rights of tenants and landlords. As a property manager, you should know those things, so I recommend you print out a copy and become very familiar with it.
I agree with Mr. Spirtos, and I do not believe the Escondido Municipal Code prohibits smoking outside non-public buildings. Most people find cigarette smoke annoying, but absent a strong factual showing of unique disturbance---i.e., your tenant is blowing smoke right into the neighbor's window---then I don't necessarily see grounds for a nuisance cause of action. I recommend contacting the neighbor to see what exactly the problem is, and working with them and your tenants to reach a reasonable compromise.