xJonnnnY wroteHe needs everything monitor, keyboard etc
He uses it for gaming and creating music
Budget is £2000 (U.K.)
I had around £140 left which could be used for a number of things :
.Can of Spray paint and some modded PSU cables in order to keep the consistent black and red theme.
. Magnetic LED's to go on the inside of the case
.Better / more monitors. I went with a 4k monitor like you asked but an extra side panel might come in handy.
. Speakers / headphones : I went the the M50x's which excel at audio monitoring for his music production but when it comes to gaming he may prefer a pair of speakers or an entirely different headset. I'll give recommendations if you'd like.
.Case fans if you find the build running hot.
Or anything else you can think of, if I were you I'd wait for another TTG member to give their opinion on the parts list, got a headache at the time of making this.
EDIT : Just realised that I marked the motherboard as purchased meaning it's cost is not taking into account for the final figure. It is £140 though so that just makes it within your budget. If you are looking to purchase some of the things I mentioned above you could probably get rid of the CPU cooler for now (from what I've heard the Ryzen stock coolers are pretty good). If you're looking to save more money lemme know!
You gave OP a 4k monitor, but slapped in a 1060 GPU? Are you expecting OP's friend to be playing 4k games at low-medium settings and still not hitting a stable 60 FPS? 1060 is not a 4k card at all. The 1070 can be argued for being a 4k entry above it's already 1440p status but that's it, 1060 is not needed at all for a 4k monitor. Unless OP's friend is slapping in a 1080/1080 Ti, 4k gaming isn't going to be a stable 60 FPS at higher settings. He would benefit much better with a 1080p or 1440p monitor at a higher refresh rate for his gaming needs.
"Can of spray paint" Uhhh no. OP's friend doesn't need to be taking his hardware apart just to get a colour scheme going. Black and Red is such a common colour spectrum for hardware it is unreal. His friend will also be spending extra money on custom coloured PSU cables, easily upwards of £70+
"but when it comes to gaming he may prefer a pair of speakers or an entirely different headset" I don't know of a single person who would choose speakers over headphones for gaming lol. OP's friend has to make the choice between gaming or music production as top priority. He can use the headphones as for both gaming and music production, he just needs to get a mic for gaming; which he should be getting as headsets have such shit quality when compared to headphones. If he's choosing music production as top quality, speakers would seem to have that better choice as they tend to have both a wider and flatter frequency response, making it easier to analyse the stereo field. If his friend was a DJ, then sure, headphones first then speakers, but if he's creating music, high end speakers are what he is going to want. If OP's friend has used these type of headphones before and knows the frequency like the back of his hand, then sure, he would be better off with them.