I have this problem to some extent with my grandparents and my mother-in-law. My grandparents have an apartment in E”Y and come for a few months a year. They tend to buy stuff from the shuk that we won’t eat, like fruits and vegetables during shvi’is or rabanut foods. We made excuses until my grandmother got the hint. We didn’t handle it the right way.
My mother-in-law is not quite familiar with hilchos Shabbos. I’ve seen her or her mother (who lives with her) lower the temperature on the croc-pot on Shabbos because it was “too hot”. We didn’t eat chulent that Shabbos. We also snuck some keilim and toiveled them.
The best advice I can offer you is if you do have to go to their house, eat only dry foods of whose hechser you are sure of. If they question you, tell him that your Rov “paskened” that you can’t eat liquid foods that were warmed up.
Honestly though, I personally would always wonder about the hechsheirim on even the dry foods. I would say something like “look, lately we decided to follow a certain Rov’s shitos in kashrus and because of that I just can’t eat at your house anymore. I don’t want things to end between us, so maybe you can come over to my house more often or we can eat out over weekends”. Then hope that you’re not left with a big elephant in the room.