I'd rather there wasn't any official policy on this, and it was just solved informally among the concerned parties.

Either Alice can abstain from replying to Bob (answering in sister comments instead, if she really needs to), or Alice can reply to Bob at the risk of looking like a douche. It doesn't seem like a huge sacrifice for either party, and not one worth escalating over.

A more general policy that would be worth having is that if some users start creating too much drama and bickering between themselves, the moderators should feel free to start nuking posts.