I have to disagree. In my experience when I force carb I set to 30 for two days, I don't vent, I just dial it down to ten for another five days. The beer is not carbed to 30 it's carbed to roughly 10. No matter if he vents, the beer is still carbed to 17 until he leaves it on the gas at a lower pressure to equalize.
If he's already carbonated to 17psi, turning down the regulator will have no impact. Where would the extra pressure go? It does not go back into the CO2 tank.
I'm sure you already know this, but when you vent the pressure, you're releasing the head pressure. Once the vent is closed, CO2 then comes out of solution to balance the pressure in the keg, thus reducing the carbonation of the beer. To have a significant impact, you would need to do this several times over the period of a day or two, but it certainly will reduce the carbonation of the beer.
In your case, after two days at 30 psi you're likely not fully carbonated and turning down the regulator to the pressure you want works. Your two days at 30 are simply speeding the process. In those first two days, your beer wouldn't be overcarbonated if you tried to pour it.
Since he's already overcarbonated, he's got to get pressure out (venting), not just reduce the pressure going in.