function wpf_allow_rss( $can_access, $user_id, $post_id ) {

    if ( is_feed() ) {
        $can_access = true;
    }

    return $can_access;

}

add_filter( 'wpf_user_can_access', 'wpf_allow_rss', 10, 3 );
add_filter( 'wpf_user_can_access_archive', 'wpf_allow_rss', 10, 3 );