Before you look at the code:
Clear the objs and the exes, and let him/her rebuild the thing.
Watch for any manual interaction (does it build via “make” alone or is there some fiddling involved).
Better yet: give him/her a naked (just bought) machine, let him/her demonstrate a checkout and rebuild. Then see how the app is started and comes up (any secret options to input?).
Then: in a pair programming session, add one or two features to the system and see where and how these are implemented.
The above may sound stupid, but I have seen projects where building alone was a nightmare, and a lot of knowledge was in the brain of the developer only. Not having a trusted build environment and having to figure out how to rebuild is a nighmare.