Which tense to use here is a style choice.

The two versions technically have different meanings, but the difference is so trivial that it makes no difference to 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.

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.