The rewrite code you posted is wrong. Your using ([0-9]+) which means match any number of characters between 0-9, so a URL like this would work:
To do what you want with the example URL provided you need to use the [a-z] characters and you also need to add something to account for the hypen.
Lastly, your URL should end with a slash like so, otherwise you need to remove the last slash from the rewrite just before the question mark.
Give this one a try:
RewriteRule ^listings/([-a-z]+)/?$ viewDetails.php?friendlyURL=$1