There is not a set regulation. It falls under your constitutional right as a US Citizen against unlawful search and seizure. It also is against the Privacy Act of 1974. Think of it this way. A prosecutor cannot legally go through an person or companies financials without a search warrant. Same thing applies to Soldiers. Just because a person is in the military, does not mean they give up their constitutional rights.
This Soldier should go speak with a Legal Assistance attorney who will provide the appropriate information to give to his command/1SG. I have posted a couple lengthy explanations on this very issue to the website.