add_filter( 'woocommerce_order_number', 'arglebargle_set_order_prefix' );
function arglebargle_set_order_prefix( $order_id ) {
if ( is_user_logged_in() ) {
    global $current_user;

    if( in_array('Administrator',  $current_user->roles) ) {
         $prefix = 'ABC-';
    } else {
        $prefix = 'DEF-';
    }
   return ($prefix . $order_id);
}
}