Interesting how not only to you sign away your right to sue but also the right of your guests to sue should they be injured in the class. As far as I know you cannot give that sort of tacit approval unless you hold power of attorney over your guest, which is unlikely. I know in my school I require even drop ins to sign a waiver directly even though serious injuries are incredibly rare (one broken leg on one of our more "intense" regular students in the 4 years I have been teaching) never an injury to a guest. So if they are not requiring your friend's guests to sign a waiver possibly the threat of a countersuit over a "twisted ankle" would be effective. That being said it would be a bluff as taking it to court could get them charged with fraud, unless of course the friend is willing to break their leg or arm for them.