Sorry for being vague:
Person A is my boss. Person A made a mistake a year ago that caused me a lot of stress and cost me financially. A couple of months ago person B told me that person A had made the same mistake with them and it had effected them negatively, and impacted their career progression. These mistakes are made through carelessness and a disregard for procedures rather than maliciously.
Person B is taking this further. This might result in person A getting dismissed. Person B wants me to tell them what happened with me last year. I'm not sure I could live with partially responsible for someone losing their job. The mistake has had no repercussions for me now. It's not effected anyone else. Person A is a nice person, we get on well. I understand why person B is angry, as I was this time last year. Should I just say that I don't want to speak to HR?