Check the faqs on the correct pH for hydro. Neutral is not ideal.
I'm not sure if all nutrients do this but mine lowers the pH to the correct level then tends to hold it fairly stable. The more nutrients the lower and more stable pH stays. I also use tap water but run in to problems using nutrient concentrations below 1/2.
If it worked for your friend then find out what nutrients he was using, how much he was applying and how often.
It's a bit of a balancing act to grow a healthy plant without pH adjustment or a pH tester. But it can be done. I can go for at least a week without any maintenance of my setup. (Other than raising the light!)
Use things like vinegar and lemon juice at your own risk. I found I was better off staying away from them.