i had a platy last week that had severe mouth rot, i was considering putting him out of his misery, but he went on his own..personally i put in a breeding net box, just to keep him from being bullied and to end his life in peace, and to monitor his eating and pooing. He couldnt eat in the end as his bottom lip dropped off too.
I feel he went quite quickly in the end, so i am happy i did my best xx
Totally up to you. x