You don't need to do this manually.
You can purge the scratch disk:
You will lose your history if you do this (all undos).
Basically you are running out of room on the system drive - not a good situation. I would talk to your friend about perhaps archiving some files and then deleting them. Or, buy a firewire drive and use that as a scratch disk.
Don't delete your friend's files if you want to remain friends.
Purging the files has worked for a while up til now, but it seems to be only a very temporary fix. After purging I can work in Photoshop for a little while, then I soon get another warning about the scratch disk being full despite that I've also moved/deleted tons of files from the hard drive and emptied the trash to clear up space. and I often get the warning popping up as I'm opening a file so there's nothing yet to purge..
Is there ANYTHING else I can do to clear up scratch disk space?? I assumed that deleting files would clear up the most space but sometimes it doesn't even help and i still get the warnings..