This game is based on the Boulderdash demo code that comes with Blitz (plus?), which isn't a problem, but some credit should be given to whoever did the original.
That's right, the game is basically Blitz Boulders, which came with the original Blitz Basic (2D), but with some new graphics and new levels. To say your mate wrote this when at best all they did was alter some lines of code here and there is a bit of a cheek and a slap in the face of the original author of Blitz Boulders ... unless they are the original author, but I doubt it.