if he is only gona play source engine games even a 600-700$build would be enough to play them at 1080p 100-200fps
But if he really wants to spend 1800$ i also included some nice peripherals in this build that are geared for gaming :)
Since the build was geared only towards gaming, i decided to go with the Core i5 4670K CPU, if you have a microcenter near you, you can get it for a really nice price too. IMO for purely gaming, it is one of the best CPU's and since yur friend will only be gaming, i will recommend it over the FX 8350/20, due to it's higer single core performance.
For CPU cooler the Hyper 212 evo will be great even if your friend decides to OC.
The mobo is rock solid and has some nice features such as audio boost, killer E2205 LAN etc...
Just grab any 2x4 GB RAM kit that is well rated... I decided to get a 2400MHz kit since it gives a small performance boost in BF 4, but other than that, anything 1333+ MHz RAM will do :)
For GPU, The R9 290 or the GTX 780 will be great choises, go whatever you like :)
For storage, IMO the best option is to go with a SSD for boot drive and a SSHD for mass storage, so that you will have really fast boot times, and relatively fast load times for your most frequently used programs :)
The case is up to you, but you can't go wrong with the Define R4 :)
I always pick at least 80+ gold PSU with budgets like these, since it is a small investment over a bronze one, but it gives you better efficiency, and they tend to be more quiet and more reliable. Also, this PSU is overkill, but it is always better to go overkill than to max out your psu :)
Optical drive is VERY optional, just decided to include one if you don't want the headache of copying all the files on another pc to get them on a USB drive etc...
The peripherals are good excamples if you need them :) I included a 144Hz monitor, areally nice mouse that i am using, well reviewed keyboard and a very well know headset :D
Hope i helped :)