Learning about the gastrointestinal system is frankly overkill, unless you just happen to be highly interested by that side of human biology anyway. Anyway, it's not just the GI system which is involved in Crohn's, it's also the immune system: Crohn's disease results from an interaction between the two. In a nutshell, your idiot immune system gets it into its head one day that the (friendly) bacteria, food, and other harmless bits and bobs which lurk in your colon are foreign invaders and starts merrily attacking your GI tract. Short of bashing it over the head with a hammer (via immunesuppressant drugs), there is no way you can 'tell' your immune system not to attack.
Anyway, if you want to learn about Crohn's, there's more than enough info on the internet to be starting off with. Just google Crohn's and various medical sites, charities, forums, etc. will come up. All the reputable medical sites will contain pretty much the same categories of info: causes, epidemiology, symptoms, medications, complications, prognosis, etc. For a basic outline they're usually pretty good. For significantly more in-depth/technical stuff, you will have to cast your net wider and be prepared to trawl through sometimes heavy-going research papers - when you can find them published in full, that is. Obviously, for personal experiences, there is nothing to match forums. You're already at the best forum for Crohn's, but there are others.
And finally, if you get really into your subject, there are books. Tens, if not hundreds, of them >_>. (Just look on Amazon.)
Good luck, and hope your friend doesn't have Crohn's too badly. (That's another thing, by the way: don't assume that Crohn's is an automatically severe disease. It is for some, but not for everybody. You do tend to hear doom'n'gloom stories online, but that's the nature of the beast: the more ill you are, the more likely you are to post on a support forum about your illness.)