lsof is your friend
By default it will show all open files, but you can get open files for a user:
lsof -u alowe (showing all open files for alowe)
or a process
lsof -p 123,456 (showing all open files for process numbers 123 and 456)
or a particular file
lsof /home/alowe/myfile (shows processes with /home/alowe/myfile open)
I do not think this always works for data files, as some applications only open the file when they want to read from or write to the file, closing the file while the user edits it... so on open, they open the file, then read, then close the file, after that when the user chooses to save the file, the process opens the file, writes out the data, and closes it.
If a file should be only opened and edited by one user the process should probably lock the file while the process runs.
I know my text editor (Kate) will open the file, and I can open it in another editor, and kate will warn me that another process has changed the file (and give me alist of options (replace, save as, drop changes), but it uses timestamps, etc to determine this, and nothing stops the second process from editing the file.
I mention these last notes because you mention text files, and they are probably not under the /proc file system because they are not technically open, just being edited by a process, but not in use.
Hope this helps