As they seem knowlegable and cognizant of Halacha, could you tell them the following.
I am recently realizing that you:
1. Warm up liquids on Shabbos in the following manner.
We are makpid not to do that and would like to continue eating by you, but have to ask you to do the following …
2. Whatever other things you need to discuss ….
Don’t present it like they are violating Halacha, but rather that you do differently, like the Cholov Yisroel example above.
The conversation may be difficult or somewhat insulting, but present it pleasantly and emphasize that you want to continue coming. And if it works when you do come, bring something for the meal as a gift so to speak, not something to be correcting them about.