The server (in this case, your coworker's computer) is in "Simple file sharing" mode, which means all incoming connections are mapped to the "Guest" account regardless of provided credentials. (This is why the username box is read-only.) However, the "Guest" account on the server is disabled (or has a password, or not allowed to log in over the network), and authentication fails.
If Administrator privileges are available on the server, this could be fixed by enabling the Guest account:
net user Guest /active:yes
If that does not help, run the "Network Setup Wizard" (on the server) and tell it to allow file sharing.
Edit: Just noticed that you have tagged your question with domain. "Simple file sharing" only works on workgroup workstations, so the real cause of this problem could be that your coworker's computer has been removed from the domain somehow. In that case, contact the network administrators.