What if the misbehaving actor refuses to extend the chain that includes evidence of its own cheating? Ie. the cheater just extends the chain at one block befor

What if the misbehaving actor refuses to extend the chain that includes evidence of its own cheating? Ie. the cheater just extends the chain at one block before the block that contains evidence of the cheating. How will clients agree to use the chain that contains the cheating proof, and not the other — just as valid — chain?