[cdud3]

I am literally saying that the role the person applied to was for a "founding engineer at a mental health startup".

This is a leading role(!) in the mental health(!!) industry. That should give some clues. They also announced that the follow-up interview as a "non traditional - a ~90 minute culture fit chat" dropping more hints.

Unfortunately he also "fail to recall the exact wording of the discussion topics".

I bet the exact wording was open enough to leave enough room to not "felt completely emotionally drained" afterwards.

I think the interviewer did him a favor. He is just not able to handle a leading role in the mental health industry in a way that would have been mental healthy for him.