The DesignData is not relevant for the IDCL files. Only the material library (so the .adsklib file that defines your current materials). Ideally you should have only one .adsklib file active (for both materials AND appearances) but this is depending on your customer setup, so we cannot be too strict with requirements.
Once you are sure that you are running on the same .adsklib as him, it should be enough to place his idcl file inside the folder where YOUR idcl files are, edit the inventor project and activate those IDCLs. At that point you should see his content center families as well.
I suspect we live in the same timezone, and probably even in the same country. If you are in trouble feel free to contact me 😉
Massimo Frison
CAD R&D // PDM Admin · Hekuma GmbH