Sherman was still a professional military man after the war, and was Commanding General of the Army from 1869 to 1883. He wouldn’t have been able to officially advise a foreign head of state of anything under those circumstances.
It was General Phillip Sheridan, who made a visit to to the Prussian General Staff, not as an advisor but as part of a military exchange. His advice was to wage total war against a civilian population. Not to kill them but to destroy all their property. He said an army should “leave them nothing but their eyes, to lament the war!” He knew something of this from his own campaign in the Shenandoah Valley.