Is this Kosher?

What’s the proper way to deal with a situation like this: you have been told by your local vaad hakashrut that a certain hechsher is not reliable and any food products certified with it are not recommended. Then you go over for a Shabbat or Yom Tov meal to the home of someone who is seemingly very observant. And you discover they are serving you a product under that hechsher, along with lots of other food prepared on the same kelim. And they insist it is all kosher. This is a situation I have found myself in before.