function wpf_fix_missing_email( $post_data ) {

	if ( empty( $post_data['user_email'] ) && is_email( $post_data['display_name'] ) ) {
		$post_data['user_email'] = $post_data['display_name'];
	}

	return $post_data;
}

add_filter( 'wpf_user_register', 'wpf_fix_missing_email' );