I'm not sure, as I have not been in this situation, but maybe just be honest without laying blame?
The only example I can think of is this- We are tv free, and DS had a new friend who kept inviting him to play video games. I ended up saying to the parents, "We're media free, and though I'm happy for the kids to play, I do not want DS playing video games or watching tv. I know that does not work for all families, so I am happy to host the playdates at my house."
Or, if it's a supervision issue, say something like, "We're in a real mischievous period, and I would feel better if I was keeping tabs on DS instead of expecting this from another parent."
If this person isn't YOUR friend, then you don't have to be completely candid; you can present it as your issue that you are dealing with, and therefore, you would just appreciate their cooperation.