That's pretty 1337 programming needed to do that - do you know what language he used?
It could well have been Visual Basic, or possibly C++.
If you entered a username, the program would have to know where that username was logged on, and if you entered a hostname, it would have to only affect that machine.
Are you sure he made it himself? That sort of thing is extremely complicated.
Master of common sense. If you don't like it, stop reading.