Importing Models from CS:S

My friend has a legitimate copy of GMod registered on his account and installed on his computer. He doesn’t have CS:S on his account, but I’ve logged onto Steam on his computer and downloaded CS:S onto his computer. However, when he tries to play TTT, most of the player models are missing and appear purple and black. If the issue is that CS:S models aren’t imported onto his GMod, can you do that manually so TTT works correctly? Or does he have to buy CS:S?

Officially gmod requires the game to be owned for the content of it in the gcf’s to be mounted.
The official solution is obviously to buy CS:S

There’s no manual set up?