The problem is that $Exploded_slug[1] is empty, don’t forget that arrays are 0-based. So you need to change your query. Also it is preferred to use lower case for regular variables like yours, so I’d also change $Exploded_slug to $exploded_slug.
Here’s the code:
$exploded_slug = explode($user->ID.'--', $url_slug);
$query = "SELECT comments FROM wp_rdp_winners WHERE id = $exploded_slug[0]";
$comments = $wpdb->get_var($query);
Actually I don’t see the point of making an array here, why not simply concatenate? Using explode here doesn’t make much sense anyway, you might want to read this simple explanation of this function with an example. So instead try this:
$exploded_slug = $user->ID . '--' . $url_slug;
$query = "SELECT comments FROM wp_rdp_winners WHERE id = $exploded_slug";
$comments = $wpdb->get_var($query);
But if you’re going to add more elements to the $exploded_slug later then use the first variant. But declare the $exploded_slug variable differently, maybe like so:
$exploded_slug = array($user->ID . '--' . $url_slug);