BuddyPress.org

Hi,
It’s a very good question.

1. When the user is deleted, he is removed from all groups
2. If the user is the creator of the group and only admin of the group, the group will be deleted. if the user is not the creator of the group or there are more than 1 admin, the group won’t be deleted.

If the group is not deleted, the site admin can always assign other users as admin of the group from management page.

Hope that clarifies.