[JamesVI]

The situation you witnessed and heard about made you sufficiently uncomfortable to create a throw-away account to ask HN for advice. That level of discomfort is sufficient to report it to HR. For all you know there have been other complaints made in the past and this incident will complete a picture of a pattern of troubling behavior.

It isn't your responsibility to gather proof or to make a career-impacting decision about these two people, that's for HR, legal and senior management to deal with, and they have the tools and processes to actual investigate. (Check your employee handbook, companies I have worked for in the past require employees to cooperate with HR investigations and refusing to comply can lead to termination). If there really wasn't any problem then HR will take no action (they don't want to get sued for wrongly terminating a manager either).