I wasn't so fortunate to see the specific episode mentioned in the aforementioned posting. One thing needs to be made very clear. Unless he is the general contracting firm, it probably isn't his responsibility, depending on the "jurisdiction having authoririty." GeneralIy speaking, "shoring" must be used anytime the depth of a "trench" or "hole" is above the workers' head.
As a firefighter/paramedic and educator, I am specifically trained in "trench" rescue. If you could see the workers' head/s then all was well by practicality or by local ordinance/law. This may not be palitable but it is the truth. As safety should always be first and foremost, stabilization of earthen walls should take place whenever possible.
Your eye for safety should be commended, but take it easy on Bob. At the same time, maybe the staff can get a "leg up" by looking into trench safety. Anytime someone becomes entrapped in a cave-in or sloughing, rescue efforts are measured in hours and not minutes or seconds. Obviously, the risk of increased injury severity increase.