Dropping below minimum once can cause an algae bloom, even if you can't see it.
Seems like once it starts, it gets ahead of normal chlorine levels very fast and then you see the combined chlorine results.
Handling it immediately makes shocking it a shorter event. Chances are if she shocks it now, maintains all afternoon and into the evening... once you see it stop loosing chlorine an hour after adding it you can try the OCLT. If it passes the OCLT at shock level (plus .5ppm or less CC) you'll be good to go and by noon or so it should be ready for swimming (sun will eat the extra chlorine).
One of the great benefits of having the FAS-DPD test is knowing something is amiss before you ever would without it, well before it gets out of hand. Handling it properly right away makes shocking much less painful.
Since I started with BBB last season I've shocked the pool three times, all due to some kind of mistake on my part. Had to shock once in early spring after a week of sun on a fallen in cover without pumps (did not do OLCT or test, just added a couple bottles of bleach and re-covered it after cleaning debris and the green on the cover was gone. I know, not right but it did no damage.), once upon opening while cleaning debris (cover fallen in again, the pool passed all tests by that evening/next morning), and once when it fell to 0 while I was out looking for my cat one day. I still don't know why a very precisely maintained pool went from being dosed to the high value the night before (then salt added), and then fell to 0 by 6pm the next day, but it absolutely happened that fast. I'm still thinking algae salt... hehe (jk). Testing the morning after adding the salt would have been a good idea, in hindsight. I probably wouldn't have needed to shock if I'd caught the loss before ignoring the pool till 6pm.
All these shocking episodes were very short. TG for a kit that actually tells me what's going on.