CUES member and board member Kim Sponem talks in this Governance+ video (use your Unlimited+ membership login to access) about her preparations for testifying before Congress. Of course, this is a little different from testifying in court, but I thought it might still have some helpful tips for your team member.
I'm going to check with an attorney source to see if he does this kind of coaching or knows someone who does. I'll post again if I get a good lead from that inquiry.
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Here's what my source, Michael S. Edwards, Attorney-at-Law, www.creditunionslaw.com, added to the conversation:
Normally a credit union's lawyer would have a preparatory session with the witness to train him or her on how to testify, but there are some courses available to train law enforcement officers on how to testify effectively in court.
Here is an article on being a witness by the Maryland State Bar Association: https://www.msba.org/being-a-witness/
Here is a link to an online law enforcement court testimony training course (there are others too):
I think Michael Gampp's point about the possibility of a witness being asked if they took a training is super interesting, too.