You could perhaps apply a rule that to downgrade the Owner status in a group with three or more co-owners, action by at least two owners should be required. That would make some sorts of malice more difficult, specifically displaing a group's original owner, but it wouldn't prevent any moderator vandalising a group in other ways.
As others have noted, this is mainly a problem for the group, not something groups.io can prevent. Just make it a little more difficult.
