This is so foreign to the dynamics of groups I've managed and been a member of that it's hard to get my head around it. But, a few thoughts which may evidence some ignorance.
1. As you describe the above, the dispute largely or entirely occurred before the group migrated to groups.io. If that's correct, wouldn't a policy about groups.io not refereeing re-existing or offline disputes be sensible?
2. As for features to make hijacking more difficult, I thought (mostly from reading others' comments and wisdom on beta and gmf) that a group with one owner can't technically be hijacked unless that owner makes someone else a co-owner, who then becomes a hijacker (downgrading the original owner to mod or worse and running roughshod). If I have that right, then one feature that might help would be creation of a "founder" level that can't be changed or downgraded or banned unless done by you/groups.io mods, etc. So, with that kind of a rubric, a "founder" that annoints someone else an "owner" would be granting equal powers except the new member's ability to mess with the founder.
At the end of the day, I'd imagine it counterproductive on a number of levels for you/Groups.io to get involved in offline issues, politics and disputes. But, I'd imagine that making a founder a "founder" could force a discontented member to start a new group if (s)he saw fit, rather than "hijacking"?