You can't boot a wubi install without the root.disk!!?
If the root.disk is being booted, but not visible in Windows, it points to some ntfs corruption (grub can find the root.disk and loop mount it, but windows cannot see it, but that seems strange). To rule that out, you can run
chkdsk /r from Windows to repair this (there's always a danger you could lose the disk in this case, or it gets 'recovered' to the
C:\found.000 folder and renamed to
file0000.chk in the process). In some cases the whole
disks directory is recovered to
C:\found.000\dir0000.chk\ and within that there'll be the
Boot in recovery mode or let the user log in as Guest and drop to a terminal and enter
ls /host/ubuntu/disks to confirm they're in wubi.
You can check where
/host is mounted to get the partition:
That will at least confirm it's a Wubi install and the partition.
They may be able to copy data from /home to /host (back to the windows host partition from the virtual disk).
You can identify the root.disk via the command:
sudo losetup /dev/loop0
bcbc@ubuntu:~/$ sudo losetup /dev/loop0
/dev/loop0: :2161 (/host/ubuntu/disks/root.disk)
bcbc@ubuntu:~/$ mount | grep /host
/dev/sda3 on /host type fuseblk (rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other,blksize=4096)
With that info you can boot a live CD, mount the /host and recover the data:
sudo mkir /media/win
sudo mount /dev/sda3 /media/win
sudo mount -o loop /media/win/ubuntu/disks/root.disk /mnt
Not sure if you can identify the root.disk from the Guest session as it's unlikely you have sudo rights, so if you can't boot in recovery mode to a root prompt, then you could boot a live CD, mount each partition and look for the file:
<mountpoint>/ubuntu/disks/root.disk. But note that running the bootinfoscript will give you the same info (and maybe also some clues if there is any file system corruption).