if you are going to sell him your gtx 980 card for only $300, i don't see why the debate as the r9 390 is comparable to the gtx 970. i will make some points anyways...
if you just want a card to install and not worry about anything, nvidia/cuda is the way to go. if you want to use amd/opencl, there are still a few programs out there that do not support it, so you would have to check those programs websites. for adobe software, the last benchmark i saw was done by a forum member. new PC - nvidia 970 or r9 390? it was an older amd card against newer nvidia cards, i noticed two things. 1, that nvidia's maxwell cards might have seen a big jump in games, but not with premiere. 2, the amd card being slower, held its own except in mpeg2.
when it comes to games, the dx12 benchmarks show amd's forward thinking designs paying off over nvidia's corner cutting with maxwell's performance for dx11. amd also has an advantage with alot of the games coming from console ports, as they are optimized for amd hardware in the xbox and playstation. most games are still dx11 and probably won't see a dx12 patch. so amd vs nvidia is kinda mixed...
the r9 390 8gb models would help with certain programs that are vram hungry, but premiere wouldn't see much use. the 8gb of vram would help with hi-res gaming, but the gtx 980 and r9 390 are only going to handle 1080p with high/max settings on graphics heavy games. playing on 1440p or 2k with medium settings the extra vram might help, but probably not enough to make up for the more powerful gtx 980.
The 980 is the way to go for adobe and gaming. Unless your friend wants to go new that is when I would suggest the 390 over the 970 due to the 8GB vs 3.5Gb vs ram.