I have this issue with a family member. It’s so much easier with a purely treif home, then everyone knows you can’t eat there- they may think you’re silly for keeping “all those rules”, but at least they won’t be insulted because they know they’re not keeping the rules. But someone who THINKS they’re keeping perfectly kosher will be insulted if you insinuate that they’re not.
In my case, we try to come at times when they’re only serving snacks (in other words, out of a package, so you know what you’re eating is kosher) and don’t eat meals there. We do make sure to always eat something while we’re there (something that we know for sure is ok, not off the dishes etc). Maybe there’s a way you can still eat certain things there?