After leading a number of trips in the backcountry I've had some experience with greenhorns experiencing this unfortunate reality.
Everyone's Body is different
Some people will need pass a BM every morning, for others every other day is normal. As a trip leader with young kids I make sure to monitor and ask about results from each of the participants. It's my position that if the subject is taboo, then those who already are having a problem will be even more embarrassed.
Using the other members of the group to help monitor if someone has gone days, or is uncomfortable can at times be a good resource as well. You don't need to ask them to spy, usually making a general conversation about the subject is enough to elicit a response.
Where to draw the line
I will allow participants to go a couple days without a productive BM, but on the third day I start becoming more concerned. This is usually the point that I take action by speaking to the person directly.
If the problem is not one of motivation (ie. participant wants to go but cannot) then generally the problem is dehydration. Proper hydration throughout the day, until urine runs clear usually solves the problem.
If not and problems persist for 4-5 days:
- Laxatives can be provided.
- Stool softeners can be provided
- Ultimately, the participant may need to be brought to the doctor to treat impaction.
If the problem is not with dehydration or diet, then it likely rests with the participant not wanting to feel exposed, or not feeling comfortable with the surroundings.
These can sometimes be the hardest to deal with, and will require one-on-one discussion with the participant about their concerns, fears, and ultimately how to resolve the problem.
Don't start off with the scared strait approach. If you start right into the..."here are all the things that could happen if you don't poop!" speech, you are likely to make them more anxious and more likely to get bottled up.
Best of luck out there!