The biggest reason for the root rot of Snake plant is most of the time is overwatering. So, stop watering immediately.
If you have a snake plant whose leaves started decaying then first of all remove all the infected parts which are rotting.
Cut away rotten and damaged areas with sharp scissors and pruner.
Spray cinnamon powder or fungicide on the cut to prevent fungi from growing.
Repot your Snake plant in a sandy soil mixture. Always prefer small pots for Snake plant.
Now, keep it in any corner where there is a medium or bright light (not in direct sunlight).
At this stage, avoid giving any fertilizer and water.
Just keep it for some days undisturbed and it will start recovering on its own.