4 of 4 Tried to re-word the question and make it more relevant to programmers since I do see some value in this question

How can I obtain feedback from external developers?

One of our external devs is leaving after successfully completing a short (1 month) project which he was contracted for.

I'd like to get some feedback from him by doing an exit interview to find out what could be better in our organization and development process.

What sort of questions can I ask to get detailed feedback from him, and not just vague responses?