His question is vague, We don't know what he will be doing with the system. If he is using it for gaming, most modern games are limited by the GPU rather than the CPU. Unless he is planning on playing CPU intensive games.
1 The most CPU Intensive Games:
1.1 Resident Evil 7:
1.2 Just cause 3:
1.3 Hitman:
1.4 Prey:
1.5 Tekken 7:
1.6 Call of Duty: Infinite Warfare Or Call of duty WW1
1.7 Rise of the Tomb Raider:
1.8 GTA 5:
But if you crank up the quality and textures and antialiasing on your favorite game, you're typically only burdening the GPU. Just as long as the CPU can keep feeding the GPU with stuff to do... if the CPU is too slow, then your GPU is sitting around waiting for things to do.
But for normal 1080p gaming, The core i7 8400 is a great gaming CPU able to handle a GTX 1050 or even a GTX 1060.