This is sort of both a teaching point but also just generally useful. If you’re on a several-day trip and you sense that at some point folks might like to wash bodies and/or clothing, arrange to stop mid-day or to just camp at a location where this works well. Do this as a group, and exchange notes on how to do so in a LNT, eco-friendly way. Minimal or no soap, certainly no soap near flowing water, dealing with waste water properly, etc.
A good trick for putting up a clothesline is to double your cord, tie off one, end, twist the cord a great deal, then tie off the other end. You can then pry open the twisted cord at any point and shove a bit of clothing in to act as ad hoc “clothes pins”.
If you stand a blue ccf foam pad on end (perhaps two of them to be stiffer) somewhat loosely it can be possible to then drape something like a poncho over the opening, pour water in it, and use that as a wash basin.
Alternatively or in addition, you could mention that some folks just don’t wash at all on trail and seem to do fine with that. How (or if) you present this sort of point of viewpoint is of course up to you, just be aware that as in many aspects of backpacking, there isn’t just one single “right” way that everyone agrees is best.