Well, that is actually a tough question! It will range from $0 (worn out converted 327) to $10,000 (std bore complete 67 MO, 1 of 200). 67s are worth the most due to the rarity, 69s DZ are running 2nd due to popularity, 68s are worth the least of the 3, except if you find the original car that goes with it.
So what your friend needs to do is document the engine numbers (casting and stamped), document all the bolt on parts that come with it. Original heads, oil pan, intake, carb, dist are worth a whole lot by themselves, but worth more as a date matching engine.
The they can post up the info and pics to these two places. They will help in determining what car it came off of and how much it is worth.
http://www.camaros.net/forums/66-whats-worth/
http://www.camaros.org/forum/index.php?board=2.0
and here are the orphan boards (trying to match up original parts with their cars)
67
http://www.camaros.org/forum/index.php?board=11.0
68
http://www.camaros.org/forum/index.php?board=12.0
69
http://www.camaros.org/forum/index.php?board=13.0
luck!