I don’t mean in the health and safety sense. What I mean is, how far would you go to cover for a coworker who’s not pulling his/her own weight because of personal issues?
When you have an office job, it’s a lot easier to be flexible. If you have a personal crisis, you can immediately talk to your supervisor, delegate work to subordinates, come in at odd hours, and work out other ways to get stuff done. If you’re in the field, you may be sharing the only vehicle with one other person, and you may be hours away from home.
I can think of a number of situations where a coworker may not be 100%, may not be close to 100%, for a long time because of their personal demons. For example, bankruptcy or other massive financial problems, substance abuse or mental health issues, chronic illness, or a disastrous family life. In these situations, a coworker may be using work as their only escape from their problems. Often, these issues carry a certain stigma and your coworker may not want to mention it, so you find out about it via the grapevine and the observations that you build up over a long time working with them.
The problem with fieldwork is that if you’re careless or distracted, you can get seriously hurt. As I mentioned before, you can’t fool around with drill rigs, excavators, and other heavy machinery. And as a consultant, you may be directly overseen by regulators, your client, and/or consultants for another company who are being paid to try and find fault with what you’re doing. So if you mess up, it may have a clear impact on your job and your health, never mind compromising the data.
Ideally, if you think your coworker has a personal problem that’s affecting their work, you’d want to bring it up with them in confidence and then bring it up with some sort of impartial HR person if you think they’re becoming a danger to themselves. In practice, if you’ve been working with someone in the field for a long period, you develop a certain camaraderie. You may not want to get other people involved who may not view your coworker in a particularly charitable light. If a coworker makes it clear that they don’t want any help handling it (or don't think they have a problem), the tendency is to not push things; to compensate for them when they have lapses, even if you’re ordinarily an (obnoxiously?) assertive person.
So how much would you cover for a coworker? In my experience, more than I'm really comfortable with. Collectively with a group of field folks, it can add up to a hair-raising number of incidents where the coworker was "protected".