...should he be sticking with an AMD graphics card or try to buy up more?
I guess it depends on how much of the budget you can fit in for the graphics card, and what resolution you are targeting. It would help to know more about what hardware this budget needs to include, and what you currently have in mind.
At the moment, I would say AMD still offers the best performance for the money in the below-$200 price range, with the RX 570, 580 and maybe the RX 590, but you could probably get something a little higher-end than those for an $850 build, assuming that price doesn't include a monitor and peripherals.
Around the $200-$250 range, I would go with Nvidia, either the 1660, 1660 SUPER or maybe one of the lower-priced 1660 Tis, which tend to be slightly faster than a 1660 SUPER, but not enough so to really be worth paying much more than $20 extra for. It's possible you might find something like a VEGA 56 on sale for around the price a 1660 Ti, but I would personally pass on that due to those cards having high power draw and heat output under load, even if they are a little faster.
If you can manage a card within the $300-$400 range, both Nvidia and AMD cards are currently worth considering. AMD's RX 5700 and 5700 XT both tend to offer a little more performance for the money than their similar-priced Nvidia counterparts, though the Nvidia RTX 2060, 2060 SUPER and 2070 do offer some additional features like hardware acceleration for raytraced lighting effects, even if its questionable whether these cards are powerful enough to run those effects effectively. I would just avoid the RX 5700/5700 XT cards that use a blower-style cooler, as those tend to run hot.