By definition, it was the משגיח’s fault. If the food was poorly coooked, you would blame the chef. If the restaurant was filthy, you would blame the manager in charge of keeping it clean. If the food is treff, you blame the משגיח.
It doesn’t help that a kosher restaurant hired non-frum people (secular Jews or goyim) as employees. And if any non-frum people had a ownership interest that is ever worse. But the bottom line is that the משגיח is responsible.