Thanks for your reply, juan. That's what I was kinda thinking too. It looks like initially the ACNPC via endorsement is free but then several hundred dollars to renew. My state only accepts ANCC certification for my ACNP, so it wouldn't carry any value other than the personal satisfaction of being certified by AACN, I guess. I just found it interesting that they would have a whole section and application process for ANCC certified ACNPs to get their AACN ACNPC via endorsement.
Good for your friend - sitting for one test was more than enough for me (I took it initally in 2002, went inactive during my time in academia and caring for family, then retook it earlier this month - it was definitely harder the second time! I hope to never take it again
P.S. For ACNP students preparing for the exam, I studied exclusively from the ANCC ACNP Review and Resource Manual. I found it to be a very good fit with what the exam ended up covering and was quite reasonable compared with many of the other study resources out there. As an added benefit, once I take their post-test, I can get 47 CEUs for studying the book. That was worth it to me and a great jump-start on my continuing ed. So just my two cents.