Built-in bookshelf questions
First off, please clarify your questions in a list, it'll help sorting through your narration! Anyway, I'll take a stab at what I think your questions are:


MDF for backing: MDF's weak point in this installation would be in its tendency to sag when supporting weight if it were to be used as shelving. As a backing it is perfectly suitable, provided it's joined properly (using pilot holes, the right screws, etc.).
are 30" segments ok vs. individual 30" carcasses? Tough to say without knowing anything about the joinery. I'm not sure why you question the ability to maintain consistent shelf height though - that's the easy part (just cut the "wall" segments to the same dimensions. 


In general, built-ins are a low-risk proposition - woodworking joinery is really meant to maintain joints subject to complex and changing loads (imagine rocking on the back two legs of a 4-legged chair). The static loads associated with built in shelves (vs a movable bookcase), even large ones from many books, are less of a concern. Actually, my main concern with this is your friend's design of the shelves to minimize sagging - make sure he has taken that into account. A helpful tool for that is: http://www.woodbin.com/calcs/sagulator.htm