EmDeer, I also found it hard to find good info on the basics of the INS. I even sent an e-mail to Honeywell asking if they could recomend some reference material but got no response. If you have some success there will you please post the results.
There was an interesting post started on Christmas eve '98 by a Ppruner called 'wondering' (who was obviously overcome by the navigational achievements of Santa in a 24 hour period). The post was titled 'INS Alignment' and has some good responses on it, although there are probably a few 'red herrings' as well so take care. Use the search function to find it in the Tech Log archives.
The book I have found most usefull is the 'FMC USER'S GUIDE' by Bill Bulfer. Leading Edge Libraries, 2031 River Falls Drive, Kingwood, Texas 77339-3113, USA. Its purpose is primarily how to use the FMS but has a few pages covering the basics prior to concentrating on the usage part.
It covers some History, Strapdown Laser Inertial Navigation/Principles, Inertial Reference Unit/s, Accelerometers Principle of Operation, Laser Principle of Operation, The Dither mechanism, Errors and their corrections (eg Schuler-tuning), etc.
Whether this is enough for your friends research I couldn't say, but it might be a step in the right direction. More info about the book is on
http://www.firstnethou.com/fmcman
Good luck!