I don't have experience in this specific area, but I was board treasurer for 15 years and these are the steps I took under similar circumstances:

1) Have your neighbor make printouts of every email and other electronic textual communications they receive or received in the past.

2) Make copies of every bit of correspondence they receive regardless of how it was delivered (USPS, slipped under the door, etc)

3) Speak to other neighbors they are close to, to see if anyone else has experienced similar treatment.

4) As soon as possible, consult with an attorney who specializes in co-op law. I know it will cost money you were not expecting, but poop is a fact of life.

What I would *not* do is take any action whatsoever before your neighbor speaks to an attorney. Don't respond to any emails or other communications, immediately shut down any verbal conversations, do not initiate any actions, even responding to the email you said in your original message your friend sent two weeks ago.

The more your friend refuses to engage, the more agitated the board will become and the more prone they become to making mis-steps. I know this is hard, but even if a board member gets up into your friend's face, advise them to simply stand there, smile, and politely say when the confrontation is over, "thank you for sharing that with me." Full Stop.

I wish you and your friend all the best.