Hi Lukesdad
I'm a Sustrans ranger, it basically involves riding the route about once a month if you can, checking for rubbish, broken glass and overgrown weeds.
Your friend will have loads of stickers to sign the route. They should be replaced if worn or missing, and should be placed when there is a change in route or possibility of confusion.
If there are mileposts these can be re-painted once a year or so.
If there is hazardous rubbish or needles etc the council should be called to dispose of them.
The Sustrans office in Bristol will supply H&S gear and litter pickers if you need them.
Some rangers organise workdays for schools or colleagues to clear sections.
A lot of people are critical of Sustrans routes, but my section is mostly canal so it's a good day out with the family.
Hope that helps.