food - Must I warn people about an untrustworthy restaurant?

What is my obligation to warn fellow kosher observant people about a Restraurant that is claiming to be "Glatt" Kosher, when in fact it has no Rabbinic Supervision at all and the owner's religious observance would not lead you to believe "Glatt" is something he cares about in his home? Does my obligation change any if at one time under a different name he had supervision as a dairy restraurant but had recently reopened in the same location as a meat restraurant?

Answer

First thing I'd do is consult a local rabbi, who may have a better assessment of the situation. Also, if the word does need to be spread that the place is not recommended, it will probably be more effective coming from him. (He's also likely to be better protected if the restaurant sues for defamation or something.)

But for theory's sake (or pretending you're the rabbi): To tell people "restaurant ABC has no rabbinic supervision", or perhaps "restaurant ABC has no rabbinic supervision, and local Rabbi X has said on the record that it's not recommended" would seem to meet all of the criteria required for constructive lashon hara:

1.) You're sure the statement is true (assuming you did your homework!)

2.) You provide only the minimum needed relevant information, in a neutral tone. (Note that I didn't include the owner's personal observance in the examples of permitted statements.)
3.) Your intention is to prevent people from coming to harm (in this case, eating food that's questionably kosher).
4.) No disproportionate harm will befall the subject of your speech. If someone will beat up the restaurant owner, that's disproportionate. If people will just take their business elsewhere because of his lackluster kosher standards, that's perfectly fair.
5.) There's no other way to achieve this goal. That's the tricky one, and why I'd talk with a local rabbi about the situation. There might even be circumstances when you'd talk with the restaurant owner first (if nothing else, to ascertain he's unsupervised), but again, that's best left to a local rabbi.

You or your rabbi might try contacting the webmaster @kashrut.com, who regularly posts alerts of uncertified products and institutions. She has a good sense of how and when this is usually done.