Depends on which engine he's using (in which case, it should have gone into either the Srouce or HL forums). If it's HL, do the above, but if it's Source, drop the model files somewhere inside SourceSDK_content/hl2/modelsrc/ and Half-Life 2/HL2/models, then run Hammer and use either a prop_static, prop_physics, or prop_dynamic (depending on how the model is compiled), to get it to appear.