MMM... good question. My experiance is..... it depends. I have already hooked a trout and broke it off returned to the same spot 45 min later caught the same fish and got my lost fly. Then again I've turned trout and they seem gun shy for the rest of the day. My best advise is to help your dad to wade a little quieter
think of it as hunting and your stalking the trout. One of the great aspects of troutfishing with a fly rod is it forces you to slow down and be observant.
I usally figure if I missed a trout or spooked it he got the best of me that day.
There is usually plenty of other water to cover.
"Nothing's Free" (Dad)