What games he play.
We are now moving to DX12 and this put us in a strange situation,
Nvidia cards are doing well in DX11 but not so in DX12. It does not support Async shader and losing out to AMD cards. In AoS with Async shader, 390x matches a 980ti. But in DX11 games, 980ti is way faster than 390x.
So what card you getting pretty much depends on what games your friend plays now and the future.
If your friend can wait, wait for Polaris (maybe Pascal as well). From what I have read from OCN, pple say that Pascal won't support async shader as well and Polaris will fix its DX11 issues.