Monitor: This build allows for AMD Freesync and 144hz. GSync is much more expensive. Which is why I went with a moderately overpriced AMD Radeon Rx 580. Overall it is a better paired value than a GeForce GTX 1060 6GB plus a GSync 144hz monitor.
Peripherals: The keyboard and mouse are higher quality gaming models.
Motherboard: The ASUS Strix B350-F is one of the best in the price range. It highlights higher quality audio than average, 8 Phase power (for overclocking) and USB 3.1 Gen 2.
Storage: The Samsung 850 Evo 250GB SSD plus WD Blue 1TB HDD will give a good balance of storage speed. So, your friend will have a fast boot drive and load a few games quickly. Then a slower hard drive for file storage and games played less often.
CPU: The Ryzen 1600 offers good value. With the stock heatsink it can get a decent overclock. A good aftermarket unit can always be added later.
I know there are more powerful hardware options. Good interfaces make a big difference in gameplay experience. What’s the point of a more powerful computer if the monitor experiences tearing? Be sure to use a DisplayPort cable to connect for 144hz.