IME cats do not like visitor cats for a very long time. Very territorial. He might want a plan for keeping them separated.
I've left my cats alone for a weekend and never had a problem or a stressed cat. I think it must depend on the individual cat.
My aunt drags her cat around and would bring her here and I have a cat and jeez, poor thing seemed miserably stressed. I always thought she would be much happier spending peaceful weekend alone!