I wish I can show you the exact post provided by NBI which was the basis for the subpoena but it is easily traceable to the person who wants to remain anonymous. Atty Diokno is also not showing this post. So instead, I will paraphrase, and I promise it's faithful to the original.
The post is as short as this paraphase: We should speak out when our money is misused or stolen. The government has money to buy a P2 billion private jet, but none for healthcare?
To drive home the point, the post attached a news item of the DOH calling for volunteer health workers side by side with the photo of the P2B high-end, high-speed, and brand new Gulfstream G280 bought in October 2019.
The person is being investigated for Article 154 of the Revised Penal Code which punishes the publication of "any false news which may endanger the public order, or cause damage to the interest or credit of the State."