The easy answer is “have the user run krfb inside the session first”. But what if you are the user, and you're already remote from the host by the time you remember you needed to do that?
You need to have configured krfb with a remote control password and any other relevant settings, first. See below, but typically you've already done this at some point, so you can move straight to the important part:
ssh you@yourserver DISPLAY=:0.0 krfb --nodialog
Then try to connect using krdc, or your VNC viewer of choice.
What if you don't have the password, etc., configured? Then:
ssh -X you@yourserver krfb
This will display the window locally. Configure and close. This mode of operation doesn't work for actually showing the remote session, evidently because the X session forwarded though SSH is somehow a new/separate session. The key to showing the remote session is passing that DISPLAY environment variable.
Other thoughts: Have a slow network? Disable key repeat, or it might stutter. Also, sshuttle is very handy if you have limited port forwarding (but it won't help the network speed situation).