Cat people love their cats. If you insult their cats, you insult them, so whatever you say, don’t say anything negative about their cats or their lifestyle with their cats. For example, don’t sniff the air, scrunch your face up, and make a comment about the smell of the cat in the house.
Whatever you do, don’t dismiss their cats altogether with a thoughtless comment about how dogs are so much better than cats. That statement will likely earn you the ire of both the cat and the cat owner all at the same time.