What is your impression of this Human Resources response to a verbal abuse case?

Actually, it does sound reasonable. They are admitting that they have never had to deal with an issue like this before and do not have clear policies in place as to what action should be taken. I would suggest that you put in writing and give copies both to HR and to the boss suggestions on what you feel should be done. Be reasonable. Firing is probably not an option unless the comments were of a dangerous or threatening nature. Let the boss talk to the people involved. I am certain that no one wants a lawsuit on their hands, but you need to give them the chance to correct the situation before you can go further legally anyway.