Have not read all the responses yet, but here are my thoughts:
If he is the target for disfellowshipping, the elders can - and will - proceed whether he collects the letter or not.
I think it may be in his best interest to collect the letter and see an attorney. Bring a copy of the letter that someone has successfully used in the past (I think it was DocBob) and ask the attorney to re-print on his own letterhead and deliver to each elder. The letter threatens suit to each elder individually. I've seen postings here on JWN from several posters who have good success with this.
For what it's worth, my mom tried to get her elders to take judicial action against my brother. Fortunately for him, one of the elders first asked her if anyone in his life now considered him to be a JW. Since he had been 'out' for 10 or 12 years, the answer was 'no'. They took no action and went to work on Michael Jackson's Disassociation. I guess they had bigger fish to fry~!!
Put a little pressure on them and they may very just back away from your friend.
Shame on the org for stirring up shit in the life of one of their own congregants - your friend's father. He certainly does not need this nasty intrusion at this juncture in his life.