The SkyRide bus service will get your friends to downtown Denver:
http://www.rtd-denver.com/skyRide/
For the remaining 258 miles, things then get
very difficult.
Culbertson is a very small village (with a population of around 600). The nearest town is McCook, which is about 13 miles away. There's just one train per day on the line betweeen Denver and McCook. This link should show the timetable but I keep getting a 'damaged file' message. (Maybe you'll have better luck):
http://www.amtrak.com/timetable/oct08/P05.pdf
If that link doesn't work, use the journey planner here:
http://www.amtrak.com/servlet/ContentServer?pa gename=Amtrak/am2Route/Horizontal_Route_Page&c =am2Route&cid=1081256321209&ssid=137
From McCook your friends would probably need to find a cab. The only public transport in Hitchcock County is provided by Hitch & Hay Public Transit. There's no website (or email address) for their services but your friends could phone them to find out if there are any convenient services for their needs. From the UK, the number is 00 1 308 276 2182.
The Greyhound website's list of locations served in Nebraska doesn't mention McCook (or anywhere else nearby), so it looks like the daily train service to McCook is their only option.
Chris