It's worth noting, there are five classifications of software in aviation - Level A, B, C, D, & E. Level A is for flight critical software - e.g. FBW and FADEC, while Level E is pretty much 'don't care' if it's not 100% correct since it has no impact on flight safety - e.g. IFE and some maintenance tasks. Naturally B, C, and D fall in between those extremes. This is all documented in DO-178, which also outlines the level of testing and documentation required for the various integrity levels.
Outsourcing Level D and E software is pretty common, since it's really not that important.
As for summer interns, the engine company was responsible for creating the FADEC s/w (although we had significant input into the requirements), so we didn't write code. However we had a lab, where we tested the FADEC software to make sure it properly interfaced with the various aircraft systems, and that our requirements were incorporated correctly. During the 747-8 development and flight testing, our FADEC lab was staffed with several summer interns, running tests that had been developed by our engineering group, and by all accounts they did a first class job of it.
BTW, most software problems in Level A, B, and C s/w were related to the requirements - either wrong, incomplete, or confusing - not the actual implementation.