Unless something has changed the Standing Rules of Engagement (SROE) are still in effect worldwide, which means a U.S. soldier, marine, etc. can take the appropriate self defense measures, so if the mortar fire was effective, the E-5 had legal authority to engage the mortar team. Judgment must be pushed to the soldier in the fight, not the TOC where staff officers can only speculate on what is actually happening on the ground.

If you want avoid the kill one insurgent, then create 10 effect, then you need to train your soldiers before you deploy them on the nuances of this type of fight. You have no right to try to control it from the TOC. This is why we have SROE to begin with, to keep chicken little from getting American fighting mena and women killed.