If he lived in New York or New Jersey, he could legally record the other person without telling them, and it would be admissible.
In states like FL, CA, PA, DE, IL, WI, NH, MA, not only would it be admissible, the person doing the recording would be breaking the law.
Now an attorney recording a client ANYWHERE and then releasing it seems like grounds for disbarment to me, but I don’t know how that works.
Also, I would have to guess that if there were legal proceedings in those specific states where two party consent is required, recordings made in states where it was actually be legal might be inadmissible in the states that prohibit it, but I am just guessing.