There would probably be an alert sent out by the developers and then they would decide whether to roll back the blockchain back to the fork point, or choose one chain and set checkpoints into the software to make sure that is the one used.
In this situation it would be hard to avoid using the longest chain, especially if it was a significant difference. Including a checkpoint in the software would be like declaring war on the majority hashrate, and they may well respond by locking out the minority until they'd got the money they thought they were owed...
This isn't entirely academic, because the majority hashrate is in an authoritarian regime that's long been propped up by a good economy, and that economy isn't looking good. If they felt at serious risk of a revolution it's not hard to imagine them turning off access to the outside world.
I think the upshot would be that you couldn't safely use bitcoin until the situation sorted itself out.