Bad answer to a bad peer-review
What do editors do when authors send them angry emails questioning the competence of the reviewer? When I edited a journal, if the author provided technical arguments against the reviewer's comments, I would forward those to the reviewer(s) and see what they say. If they didn't, I would usually say something generic about our faith in our peer review process and apologize that we cannot accept the paper. Either way the dispute does not usually escalate beyond the three of us (author, editor, reviewer).

Filing a lawsuit would be unprecedented. Banning you from submitting is more conceivable, but still unlikely. After all, it's to their advantage that authors submit potentially-publishable papers there. If it happens my guess would be that the author wrote something that crosses into the realm of personal attacks: for example, "tell your reviewer to take a gun and shoot himself".

In any case it is not something to worry too much about - after all it's not you that sent the bad answer - unless the author did write personal attacks, in which case you could send the editor an email distancing yourself from the author's behavior.