Tenant sleeping in windowless room

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I have a tenant in my 2 bedroom property. She lives there with her son. 

There is a master bedroom on the main level and another bedroom in the basement. The basement also has a few storage spaces/offices etc. My tenant has put her son in the basement bedroom and took one of the windowless storage spaces downstairs as her room. The master bedroom upstairs is being used as an office/playroom. 

Is this legal? Do I need to tell her that she can't be sleeping in a windowless room? Or do I have her sign some kind of waiver in case something happens? not sure how to address the situation.

Thanks!