From the computer management checks as the partitioned USB stick, and what type of file system uses. If all goes well, and the file system is NTFS or FAT16/32, a utility may be sufficient to control disk usage (WinDirStat, or Sequoiaview) to find a lot of hidden files, the program used to access, bypassing the forms of limitation of Windows (as long as they are not viruses, or it may contain).
In the latter case you should recover any files placed in the quarantine folder of the virus that he has on his computer, hoping to find what he needed.
If the file system is incorrect, or inaccessible, you should use a control program of the partition to allow for any necessary exploration of the strange partition, and data recovery.
Surely you need some experience and caution in what you can do.
Finally try to educate your friend to affordable backup and recovery strategies: no wasted time, and probably you will not lose any more time and he will not lose more days of work to recover lost data!
if all of the above fails then try this:
Data Rescue 3 for Windows