Trusting a Person who is Mechalel Shabbos Befarhesia
A person whom we suspect of violating a specific mitzvah does not have ne’emanus, trustworthiness, regarding that mitzvah; one who desecrates Shabbos publicly loses his ne’emanus regarding all mitzvos, just as, lehavdil, a non-Jew. When it comes to matters of kashrus, a mechalel Shabbos befarhesia does not have any ne’emanus that the food being offered is kosher. Even if kim lay begavay, we know for a fact that the host is truthful, one should be machmir, strict, and not eat the food. However, in case of necessity, if a person is very weak and does not have another option, he can rely on the host’s integrity and eat there. Similarly, there is room for leniency if baalei teshuvah are being hosted by their parents, and they know that they can be trusted to serve them only kosher food.