Unfortunately, there is no clear-cut answer and it is always a facts-and-circumstances thing.

Find out how much activity your client does towards the rental.  If the just collect rent once a month and pay the mortgage and taxes, no.  If they do lots of work and planning and repairs and more, maybe.

As a general guideline, I reference the 250 hours for the Safe Harbor.  How far away are they from it?  If a total of 200 hours is put in, then it may be a good case to be called a business.  If only 50 hours are put in, probably not.