bp_groups_update_orphaned_groups_on_group_delete( BP_Groups_Group $group )

function bp_groups_update_orphaned_groups_on_group_delete( $group ) {
	// Get child groups and set the parent to the deleted parent's parent.
	$grandparent_group_id = $group->parent_id;
	$child_args = array(
		'parent_id'         => $group->id,
		'show_hidden'       => true,
		'per_page'          => false,
		'update_meta_cache' => false,
	);
	$children = groups_get_groups( $child_args );
	$children = $children['groups'];

	foreach ( $children as $cgroup ) {
		$cgroup->parent_id = $grandparent_group_id;
		$cgroup->save();
	}
}