I would let it die by not commenting or giving it attention. Posts that get attention can also due to algorithms be more visible to more people. In my opinion people who slanders others will often see that it backfires as not many will be aquainted with such people, who knows who will be next to feel their rage.
What you can try is to report the post to Facebook. It rarely works as I think Facebook will also take into account freedom of speech, but sometimes reported posts will be taken down.
You can of course also reach out to the host, tell them that you have been informed that they have said this and that online and ask if they can inform you more and also that you wish they had contacted you directly.
If a THS-sit - take screenshots etc as Maggie8K suggests.
Edit: if you at all want to contact the host, in my experience people think they can say anything online, but if one pick up the phone and call them, they are rarely confrontational and often they will back off. It is interesting how many can’t stand up in person for what they find appropriate to shout out on social media. 
