It would be helpful if you mentioned on which OS the renaming was done. I suspect it was on Windows OS, which is the OS that is mot likely to give case-sensitivity issues with files and folders renaming.
When you say that after the case-renaming was pushed to GitHub, and your colleagues clone it but found the folder to be named as before (i.e. lowercase), do you mean a fresh clone, from scratch? That would be rather odd, since if the changes are effective on GitHub a fresh clone should be verbatim, whereas pull operations might not be effective under Win OS (I don’t know about macOS, since I don’t use it).
Win 10 Supports Case-Sensitive Directories
It’s worth mentioning that now, in Windows 10, it’s possible to enable case-sensitivity for entire directory trees:
When working with Git on cross-platform projects, it’s a good idea to enable this native feature on the repository directory tree, since it reduces the problems associated with same-name different-case renaming operations.