$2,000 is a decent budget for an upper mid-range build. He'll need a good monitor and peripherals to go with it but those really do add to the cost especially a good monitor to really get the most out of the graphics card. I recently had to replace my PC with a £1,400 pre-build but thankfully I already had £500+ worth of hardware recently upgraded before I bought just the unit itself.
For $2k, he'd be looking at an i5/i7 with a high clock speed 4+ GHz total, GTX 1070/1080, 16gb DDR4 RAM, an SSD plus HDD and a decent cooler. Graphics card prices are mental right now so he may even be best going for a solid pre-build where the price hasn't been bolstered to accommodate the individual hrowing prices of cards. This is what I had to do - spend £1400 on a pre-build or £1,700 with the components separately - usually it's the other way round!
In terms of a monitor, a high refresh rate is key to get the most out of the card. Don't want to be rocking a 60hz vintage with a 1080ti for instance.