I concur with Mr. Schutz, but I'm curious as to why you believe her smoking in the unit would be "too difficult" to prove. As the landlord, you should have (a) reserved the right to enter to inspect the unit for breach conditions and (b) probably smell cigarette smoke outside, regardless. Your own testimony of what you've experienced would be competent, probative evidence that your tenant is breaching the lease by smoking indoors. You don't need to recover physical evidence, or catch her red-handed.
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You could send the tenant a Notice to Ceases (that the Tenant is in breach of the lease) or a Notice to Quit (that you want the tenant to vacate at the end of lease) If the tenant is month to month you can server a 30 day notice at the begining of the month.