Question: Is there any way to see all publicly shared Google Drive items/documents from a specific Google user?

Answer: 
I wrote a Google Apps Script macro that you can attach to a new spreadsheet (or an old one, if it would make any sense) that accomplishes this task. To attach a script to a spreadsheet, go into that spreadsheet (or create a new one), click "Tools"->"Script editor...", copy and paste the code into the editor, click "Resources"->"Current project's triggers," select "onOpen" under Run, select "From spreadsheet" under Events, make sure the third drop-down menu reads "On open," then save the script under a name of your choice.

The code below:

1. asks you to enter the email address of the user for whom you want to get a list of shared files
2. creates a new sheet within the active spreadsheet
3. populates it with two columns of data: one with the names of all of the *publicly shared* files owned by the user you specified AND live hyperlinks to those files, and a second column containing the names of the folders containing these files

Here's the code:

```
function doGet() {

  var owner = SpreadsheetApp.getUi().prompt("Enter owner's email address:").getResponseText();
  var ssheet = SpreadsheetApp.getActive();
  var newSheet = ssheet.insertSheet(owner);

  newSheet.getRange('A1:B1').setValues([["File Name","Containing Folder"]]);
  newSheet.getRange('A1:B1').setFontWeight("bold");
  var cells = newSheet.getRange('A1:B500');

  var files = DriveApp.getFiles();
  var ownerFiles = [];
  while (files.hasNext()){
    var file = files.next();
    if (file.getOwner().getEmail()==owner && file.getSharingAccess()==DriveApp.Access.ANYONE) {
      ownerFiles[ownerFiles.length]=file;
    }
  }

  for (i=0; i<ownerFiles.length; i++){
    cells.getCell(i+2,1).setValue("=HYPERLINK(" + ownerFiles[i].getUrl() + "," + ownerFiles[i].getName() + ")");

    var folders = ownerFiles[i].getParents();
    var folder = folders.next();
    cells.getCell(i+2,2).setValue(folder);
  }
}

function onOpen() {
  SpreadsheetApp.getUi().createMenu('List')
                        .addItem('Public Files Owned by...', 'doGet')
                        .addToUi();
}

```

To run the script, reopen the spreadsheet after first saving the script, then click "List"->"Public Files Owned by...".

Something to bear in mind is that this is an inefficient script; if you have a lot of files stored in your Drive, this could take a *long* time to execute. In addition, it is currently written to accommodate up to 499 entries. (Increasing this maximum is simply a matter of raising the number for the 'B' column in the `cells` declaration.) It is also currently written to output only the first parent folder if a file is contained in multiple folders.
If you are admin you can use the Drige API with a service account impersonating that user.