$.ajax({
type: "POST",
url: <?php echo base_url().'index.php/user_management/manage_users/ViewProfile/'?>"+JSON.parse(item.UserID),
success: function(output_string){
$('.second_column_content_container').html(output_string);
}
});
URL の結果を一重引用符または二重引用符で囲む必要があります。例えば
'url: <?php echo base_url().'index.php/user_management/manage_users/ViewProfile/'?>"+JSON.parse(item.UserID)' or
"url: <?php echo base_url().'index.php/user_management/manage_users/ViewProfile/'?>"+JSON.parse(item.UserID)"