Speak to people involved in kashrus for wine, the ones that do production for a few months at a time. They get "bochrim" to do the work.
All I know is that if you know someone is not shomer shabbos, even if they are careful not to do anything in public, their kitchen is not considered kosher. There are heterim, but you don't need a heter for a shomer shabbos...
Regardless, we're not getting into the halachic technicalities. Someone who isn't keeping shabbos, even in private, is not "on the derech."