So enoon, from your post, are you saying that IF "off on a frolic of his own" is a legitimate legal term and recognized in the US, that it's use in the letter of revocation for the NW pilots certificates that the FAA has released NW from any liability in this case and placed all blame on the crew, all in one letter?
No, I am not saying that. I have no knowledge of the relevant US Law, but I would be surprised if the letter purports to make a determination regarding any responsibility on the part of NW for this particular piece of conduct by its employees. Most lawyers trained in "Common Law" systems (England / the USA / many countries formerly within the British Empire), would see the phrase and associate it primarily with the broad idea of an employee doing as he pleases, rather than what he is supposed to be doing. Like much legal language, the phrase has connotations extending beyond its original precise context and significance.