Rent out rooms in a 'dwelling only' property?

The property is listed as 'dwelling only - 2 family' and the landlord rents out rooms (8 rooms, he doesn't live there but some of his family members stay for free) in Kings county, brooklyn, ny. Is that legal? I would like to visit my friend for a week or two and she is afraid her landlord would charge for the extra person...we're just looking for a mini loophole to exploit temporarily. Does the law allow me to visit her or is it just what the landlord says it is.