How do I transfer Dwarf Fortress saves from a PC to a Mac?

The saves are located under the ~/data/save directory where Dwarf Fortress is installed. Grab the current/ and region[X]/ directories from your friend's computer and put them in your ~/data/save location-this will also contain the world.sav file (no rebuild necessary).

Note that if you already have a fortress on your installation, you may want to copy your current/ and region[x]/ folders to another directory (e.g. Backup fortresses) - I'd highly recommend this anyway because sometimes the game can crash while saving and corrupt your data.

Note: the [X] in region[X] will be a number, depending on how many places your friend has rendered