Our tenant at will has recently been having an overnight guest 2-3 times per week. He brings his dog on these days.
We verbally had agreement with tenant that there are no pets allowed. Since it is not her pet, can we tell tenant that pets aren't allowed even if it isn't her dog? I think this pattern is going to continue and want to know if landlord has right to tell tenant at will no dogs.

Prohibit pets in apartment
Very good. Your question reminds me of the old Pink Panther joke ("does your dog bite?"). Anyway, a landlord can prohibit pets in a rental unit. The prohibition will extend to all people and visitors under the tenant's control (i.e., all visitors). You do not have the no pet clause in writing, however. I would ask the tenant to enforce the verbal no pets rule. If she refuses, rather than trying to take her to court to enforce your oral agreement (something that is bound to foul the relationship in any case), just give her a thirty day notice to quit and find a new tenant. Don't tell her you are evicting her because of the pets. You are not required to give a reason. Good luck.