[wlug] Full partition. Any ideas?

Hi all I have a friend with a problem. His Ubuntu system (2.26.32-x) won't boot. Booted from CD. Gparted showed partition partition size (ext3/ext4) is 120GB and 100% full. Disk analyser only shows 20GB in files (though 120GB on partition). Robbed 2GB from swap file to add to partition, but partition still shows 0% free. Can read files in the partition, but have no idea where that 100GB has gone. Since disk has another unused partition, plan of attack at the moment is to install 10.10 in the unused partition and recover data as needed. Anyone else seen this problem before? Any suggestions? Michael