Professionally, written with only the facts listed above (No need to further your explaintion, as that is your privacy)

I would email them immediately before situation gets out of control.

If this grows into something bigger - can deal with it then.
At this time it is Not the Mangements problem - BUT those that had nothing to do with their time - and chose to start a problem

If you can try not to get upset, as I said that person has no life and so miserable in themselves that they want to bring people down with them.

This really should take care of the problem