In reply to GDes:
The walk in via Mosedale is great with lovely views of the "wild side" of the Scafells in clear weather. During really wet weather it can sometimes be a bit problematical crossing Lingcove Beck but that shouldn't be an issue at present.
There are plenty of great camping spots around the Great Moss so they just need to arrive well before dark and have a look around. It's a very atmospheric place to camp with a feeling more like a remote part of Scotland than the Lake District. It was the site of my very first wild camp back in the 1980s.
For drinking water just pick one of the tributaries that run down off the valley sides to join the Esk. As long as there is a good flow and there are no dead sheep in the water upstream there shouldn't be any problem.