Although I wouldn't change a prototype based purely on the feedback from the first tester, I would definitely and fearlessly make such a change if I got a majority of the first 4-5 testers revealed a problem. ...and I would stop wasting testers on the old design until that change was complete. Here is why...
In UX, Ignorance is precious. Every user only has one first impression for a given prototype. Once they have used it up, they will never again approach that prototype as a complete innocent. Informed feedback gathered from experienced users who have had time to digest and consider their responses to your design are practically worthless compared to the in-the-moment response of a new user, seeing it for the first time.
If you have an infinite testing budget, then go ahead and waste your tester's ignorance; you can always buy more testers. If, on the other hand, you live in the real world with the rest of us, then horde your first impressions like a miser hording gold coins. Spend them as slowly as possible because you probably won't get any more, once they are gone.
There is a spectrum to appropriate response to tester feedback. Redesign everything that each tester stumbles over and you will never finish the project. Wait to redesign until all of your testers have gotten a look at the old design, and their treasured ignorance will be gone forever.