I'd say this depends on what card he wants, whether he is selling his 670 to off set costs, and if he is OK buying used. I would say that if he could pick up a 7970 for ~$100 after selling his 670 it would be worth it. Sure the 670 is capable, but the 7970 is seconded only by the Titan. It's faster than a 680 and a 680 is definitely faster than a 670 so...
Now if he were looking to outright pay full price out of pocket for 7970 then I would definitely say no. That doesn't seem to be the case though. When I made the jump from a GTX 580 to 7970 I sold my 580 for $200 (could have made more but I didn't mind a quick sale) and paid $130 out of pocket for my 7970 I now have. $130 for a difference between 30FPS and 45FPS is an awesome deal. Can't argue with that!
Plus the hardware is just better...especially the 3GB of RAM if he wants a 1440p monitor and/or mods his games (Skyrim can quickly hit 2GB+ with mods).
I am however a PC enthusiast. In my mind just because my hardware may be "fast enough" doesn't mean I don't always want it to be faster. If he can comfortably afford to go to a 7970 then WHY NOT DO IT?