Definition. Under Law 41, subsection 3 of the Laws of Cricket, the ball may be polished without the use of an artificial substance, may be dried with a towel if it is wet, and have mud removed from it under supervision; all other actions which alter the condition of the ball are illegal.
I think sanding the ball comes under “all other actions”
As the captain to decide to do that is shocking and then to get the newbie to do it is even worse
He cant be trusted to captain anyone again after that and while he is in the team he will undermine every captain which he has done all series tbh with him moving the field and telling the bowlers what to do