I don't think letting your 3 day old CIO is even "acceptable" in the mainstream these days. Most of the mainstream books I've read (okay skimmed) don't recommend CIO until AT LEAST 4 months.
Are they aware that virtually no one who has any qualifications to advise parents on baby care would actually support CIO on such a young baby? ie. are they just winging it (not that that is necessarily bad) and have assumed that if they were planning to CIO at some point that they might as well start as early as possible? If that's the case maybe someone, anyone could get them a book. Even if it's a super mainstream book, and not one of those crazy hippy books that some of their relatives read. Even Ferber himself would not suggest CIO on a newborn!
Or perhaps they've got a hold of Babywise or something. Yikes! Maybe they could be convinced to at least google Gary Ezzo or whatever they're using.
"Joey's mom" I know you probably can't do these things but I just needed to get that off my chest! Thanks for listening.