The former prosecutor's name is Mark Pomerantz.
One very interesting item from his Wikipedia biography that caught my eye was this:
Pomerantz wrote a book about his involvement in the investigation of Trump, titled People vs. Donald Trump, which was released on February 7, 2023.
I would think the guy effectively surrendered any claim he may have had to quash a subpoena related to his work as a prosecutor when he published this book while a grand jury was considering criminal charges in the matter he was investigating.
That book is the whole reason he’s being subpoenaed.