I would check all of the following
1. If the file is blocked (typical when u download things or pass by shares or sessions). Just right click and go to properties.
2. Check if the file is somehow protected and unprotect
3. Check Word version : yours and the one of your colleague. If these are different then be sure to have them in the same version and build. Exactly the same.
Once checked all of these points, if still not working then I would suggest to fall back to uninstall completely office from your colleagues client and install this back from scratch.
