I thought that opening someone else's mail, which this person did, was a federal offense.
From google "It's a federal offense to open mail that doesn't belong to you. If you do it intentionally, you could be looking at a $250,000 fine and up to five years in a federal prison."
The person will most likely be charged with a federal offense. USPS does not play with this, they will presses charges even if the person's who mail was stolen does not.