Well Ruffy from what I gather, an Err 99 message could relate to the card being used, the lens, camera or even grip. Your friend could try the following to isolate the problem. It was offered by Canon in respect of a problem with a 10D some years back but I think it is applicable to a 400D as well.
1. Turn off the camera.
2. Remove the lens, battery, and card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert a fully charged battery, and turn on the camera.
Does the "ERR 99" message appear? If it does, then the camera should
be serviced. If it does not, then please proceed to the next set of
steps:
1. Turn off the camera.
2. Insert the card.
3. Turn on the camera.
4. Format the card in the camera.
Does the "ERR 99" message appear? If so, then the card is the most
likely source of the issue. Try using a different card.
If the message does not appear, please complete the following steps:
1. Turn off the camera.
2. Clean the lens contacts by gently rubbing them with a soft cloth. Be careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
If the "ERR 99" message only appears when one particular lens is
attached, then that lens should be examined by a service technician.
If you see the "ERR 99" with a different Canon lens attached, then the
camera should be serviced.
If your friend uses a grip, this should be checked as well by Canon if all of the above yield nothing.
Hope this helps