Instead of using spaces consider to use non-breaking spaces \xa0. The following code uses String.prototype.replace and a regular expression to replace all the spaces by non-breaking spaces.

function myFunction() {
  const message = 'foo        bar'.replace(/ /g,'\xa0');
  SpreadsheetApp.getUi().alert(message);
}

Instead of using spaces consider using non-breaking spaces \xa0 instead. 

The following code uses the String.prototype.replace with a regular expression / /g ^* to replace all spaces with non-breaking spaces.
_{* Global flag g is required or it'll only replace the 1^st space.}

function myFunction() {
  const message = 'foo        bar'.replace(/ /g,'\xa0');
  SpreadsheetApp.getUi().alert(message);
}

If using this repeatedly in your script, you could also create a function and call it where needed:

function myFunction() {
  const message = noBreakSpaces('foo        bar');
  SpreadsheetApp.getUi().alert(message);  
  
  function noBreakSpaces(txt) {
    return txt.replace(/ /g, '\xa0');
  }
}