It depends. A resistive voltage divider only provides a constant voltage if the load that it is connected to draws a constant current. It has to be designed with a specific load in mind. If whatever the Arduino is doing results in it using a relatively constant current then it could work. But it might not work well for another Arduino project that has different current requirements.
It's a poor design for a power supply. So poor that it can hardly be called a power supply. Even a Zener diode would be preferable. Attempting such stunts will result in your EE friends laughing at you.