function exclude_completed_course_from_stale( $tags, $user_id ) {

    $stale_tag = 'Inactive Customer';

    // If the user has the 'Completed a Course' tag, remove the 'Inactive Customer' tag from being applied
    if ( wpf_has_tag( 'Completed a Course', $user_id ) && wpf_has_tag( 'Customer', $user_id ) ) {
        $key = array_search( $stale_tag, $tags );
        if ( $key !== false ) {
            unset( $tags[ $key ] );
        }
    }

    return $tags;
}

add_filter( 'wpf_apply_tags', 'exclude_completed_course_from_stale', 10, 2 );