It sounds to me like they went to debug from IDE, and expected the assembly to be loaded for them... They either need to manually NETLOAD, add an autoloader registry key, or (what I do for debugging is to) load a Script from the Command Line Arguments in the solution's properties as the 'debug' Drawing1.dwg is located in the ..\bin\debug\ folder.
Works well while in IDE, yet leaves no 'residue' in the registry when I build for release. Lemon squeezy.