An illegal order?
He was ordered by his superior not to divulge information - an order that his superior had every legal right to give and which he had every legal obligation to comply with.
An illegal order is an order in which a superior instructs a subordinate to violate the laws of war.
I am unaware of any provision in the UCMJ or the Geneva Convention which makes it compulsory for a officer to immediately report the circumstances of a soldier's death to his family.
The Army, as a matter of policy, traditionally waits until a uniformed service member can contact the family in person.
If not telling the family immediately is illegal, the Army has been routinely breaking the law for centuries.
There is a lot more too this story (and we may never know the truth) - if I can find the article, I’ll post a link, but there were soldiers who were told not to reveal what they knew - that constitutes an illegal order when you have an ongoing investigation. It’s very close to tampering with witnesses (and some legal types would argue that it was).