I would say that one GPU is always better than 2 (most of the time). What will your friend be doing? Im no expert on GPU's but I have some knowledge of them.
the dude wants to spend 500 bucks...get a 7970...369 on newegg. and don't go playing games on a 32" TV...input lag will be horrible and the pixel density will be shit...
Depends on his intentions. If he only desires to game I would recommend a 7970 (provided he has a CPU capable of complementing this graphical beast) and save the extra $100 and upgrade another part of his system if needed; such as adding an SSD, extra Hard Drive, more fans or a better CPU cooler. If he also desires to utilize his machine to edit videos in Premier, go with a 680, he will adore CUDA; while still getting near identical performance to the 7970. However do bear in mind that selecting the latter will raise the cost an extra $80, leaving him with only 30 spare bucks to upgrade other said components.