Don't reply to them, it will feed the fire.
It's also not your business, as you say. Especially if they were sending the messages about you to your friend and he was passing them on to you.
Have they involved you because you have responded to them in the past? If so, I can understand you wanting to defend your friend but you are not doing him any favours, just winding them up even more.
Goodboy is right, he should keep a detailed record of any contact, any abusive messages or comments made in person or via Facebook etc. Times, dates, names, copies of messages or screenshots with names etc.
Keep it all, build a case for harassment, but don't reply at all, no matter how difficult that may feel.
He can speak to Citizens Advice and Victim Support have this to say about harassment.