The company I work at on various campus's requires an annual safety training class. Even though I don't work in any confined spaces, they have lots of material on the subject in part of the training. I have to learn and pass a test on it all.
They discuss using gas detection monitors and they have to be on a selfie type stick that can be easily lowered to floor height and not just check air at elbow or waist level. Having a tether/harness on the person that enters and they have a recovery system (pulley/tripod system usually) so they can extract him without any one else entering if needed, there must also be a person attending the recovery system and in full contact with the person in the confined space at all times.
A lot of this is in place since 2 guys were killed many years ago just in a trench about 6 feet deep, the first guy bent over to do something and the gas (I don't recall what it was) knocked him out, a few minutes later another worker saw him and went down to help and he was overcome by the same gas.
Seems there should be some sort of training or at least a publication available for all the farmers and works that may have to deal with these manure pits. Can they be built without any state/city/county inspection? The inspection could cover this training.
The manufacturer of the pit/pumps/etc should have warning decals on the equipment, maybe it already does, I don't know.
With them being in their thirties, I would think they would have known about the potential hazard, but maybe not.