The primary idea is for Sol to just act within Mitsuru's brain alone, essentially nullifying Red's control. The stone has been growing in our brains all since camp started, so it seems possible that in the final week, it's big enough for that...

[ But if that doesn't work, holding Mitsuru back from their end would be another option. One that Boa is also less likely to trust for, uh, obvious reasons. ]