I upgraded to a Freesync 4k Samsung monitor when I still had my R9 290 Tri-X. It was fantastic for many older games, but ran quite hot at full tilt even in menus. The 4GB of memory became a bottleneck in newer titles...
Even in older games like ARMA 2 and Sins of a Solar Empire the GPU would be forced to raise fanspeed to audible levels (this Tri-X was silent with 1080p).
I got a 2nd 4k monitor (now just need a larger 4k120hz for the middle ), this time a Gsync Acer 4k monitor while upgrading to a GTX 1070 and I was surprised at how well it handles 4k. Every AAA I own except for R6 can maintain a smooth 16.6ms on a mix of High and Ultra settings with AA off.
I just got No Mans Sky and was pleasantly surprised at how well it runs the game at near-Ultra settings except for lowering shadows to high and having no AA. Even Elite Dangerous, a game that couldn't be maxed out at 1080p with my R9 290, can now be maxed out on 4k with my 1070 at 4k60. I found raising the frame limit above 60hz would actually give me smooth 16.6 ms frames (because gsync has all the control). I didn't expect such a dramatic performance leap. That kind of performance was what I expected the 1080 ti to bring me. E: D is easily my most played game. The fact that the 1070 runs it so well means I don't need a stronger card like I was expecting.... GTA V is the same thing. My R9 290 can't do 1080p60 Ultra while my 1070 can do 4k60 at near Ultra (shadows lowered to high, no AA)
R6 Siege is the only title I play that forced me down to medium settings (except for textures, AF, etc). I see this as a sign that the 1070 will be a low-medium settings 4k GPU in a year or 2 for graphically intense games. But that is far better than I hoped for.
If your friend is truly looking for a GPU that can do 4k ULTRA (nearly Ultra and no AA), then I would easily choose the Titan X. If he/she is not someone who cares to max out games or plays older titles, a gimped GTX 1070 can provide a great gaming experience at 4k for a few years. I plan on upgrading to a 1080 ti or Vega, so getting the 1070 over the 1080 saved me a lot of $$$.
And keep in mind - Pascal drops its clockspeed by a 12 mhz step every few degrees over ~56C. This means that if you like a quiet GPU AND want to maintain its max clockspeed, you need a quality cooler or water.
To have the highest stable overclock, you must keep the card cool. 2.0 GHz minimum base clock requires a much higher core clock offset on air vs water merely because of the higher temperatures.
I consider Pascal to throttle itself after the first clock step is reduced after ~55-57C or around there.
For example: if I run my GPU at 100% fan speed, I get 2038 Mhz all day . But when I go back to the near-silent fan curve, my temperatures increase and I get 2001 MHz minimum base clock with it flickering 1-2 steps above at times.
Therefore, the Pascal Titan X stock cooler is despicably terrible in my standards, especially for the price. Watercooling or an aftermarket open cooler is the only option IMO. If your friend doesn't want to get their hands dirty, I would have them wait for the 1080 ti. Most likely your friend is normal and won't care about clockspeed. He/she will probably be content with a Titan XP, even if it was stuck at 1200 MHz. I bet a peasant-class GTX 1060 6 GB could even scrape by (low settings look alright at 4k) for the wait for the 1080 ti. I would love Titan XP performance if I could have it packaged in a quality air cooler that can maintain nearly its max boost bin at a near-silent fan curve. I can't argue that it is wrong to be content with reduced performance. But if my Pascal GPU runs above 70C, I would have to increase my OC even more to maintain my desired performance.
Also keep in mind 4k120hz is HERE but pricey! If your friend likes high refresh rates and wants to blow $2k+ on a rig, I would again wait and cheap out on the GPU until 4k120hz is affordable. 4k60 is really cheap now and I consider it the new 1080p. 4k120hz is what I would consider spending $1k+ on a GPU for. Titan XP would choke at 4k120hz Ultra so why not wait for Volta if you're gonna splurge over a G on a GPU. Titan XP will make a great 4k60 Ultra card but will struggle at 4k120 Ultra; It will essentially perform like the 1070 performs at 4k60. If your friend would be content with that, then the Titan X would be a good choice.
edit: We all have different noise tolerance levels. Any GPU fan @ 100% speed is intolerable to me, even with headphones. I need my MSI Gaming's fan speed to be below 50% at its highest. This means I cannot maintain 2.1 GHz/9.6 GHz with the stock fan profile. I don't want to hear my PC ever.