By the way the nature of the Priest bug means unit tests would not have detected anything. Individual software modules were working correctly. In that case a integration test system would have been needed to have any chance of detecting the edge case problem with the interaction of separate systems.
Sorry, you're correct on that - have a bad habit of saying "Unit testing" when I mean "testing"
As for the rest - if they are able to learn Doctrine, they are able to learn simple Unit Testing. There comes a time when you need to keep the bar at a certain level, or end up going backwards all the time.
There's a reason the Peace Corp doesn't accept most applicants - well-intentioned as they might be, a volunteer who doesn't know anything does more harm than good.
If they don't know how to code php, they should get involved in other ways. You can say that sounds harsh, but you wouldn't accept volunteers to build bridges in Africa just because they have good hearts, right? They should actually know something about bridge building.
If they REALLY want to be coders, then tell them to learn how to code unit tests first. Will make them more useful to the world in general. Being a coder is not 100% about writing code.
EDIT: for those interested in learning more, here is a decent definition of some different types of tests:
http://programmers.stackexchange.com/questions/214529/how-to-create-unit-integration-tests-for-my-web-app
