Which tense to use here is a style choice.

The two versions technically have different meanings, but the difference is so trivial that it doesn't affect what someone would understand.

>If he had sued, you would have had to prove that you **didn't discriminate**.

In this version with *simple past*, the referring time is now, and the interview happened in the past, so it means **the interview happened before now**.

>If he had sued, you would have had to prove that you **hadn't discriminated**.

In this version with *past perfect*, the referring time is the imaginary time when the interviewee sued, which is in the past, so it means **the interview happened before the suing**.