Thanks all for the different perspectives!
I am the host in the story and purposely ask the question from the guest POV as I didn't want my views on the matter to influence your answers.
Honestly I found myself put on the spot yet again. I don't like their house mate, he makes me very uncomfortable. Sit there, don't talk and chain smoke. Great guest!!
I'm pretty sure it's my pregnancy hormones and sense of smell that makes me dislike him though. He stinks of cold ciggies.
I was pondering how to say no, and wasn't sure how to. But then my guest wrote to me again saying that their house mate had other plans anyway.
From now on I just won't invite them to diner until their house mate has left! No more awkward situation.