TX is a good PSU and that wattage is pretty nice, but I'd recommend you get HX 750 instead, for the CFX setup with those two particular GPUs. They are quite power-hungry and you would need that little extra.
Barely. What you lack are enough plugs to power both cards. Might want to upgrade the PSU to something that was intended to power two dual 6pin PCIe plugs. I think two 7950s stock are looking to pull 400W just for them. You should be looking at around 550W for the complete system with no OCing. You've got 636W on the 12V rail on your PSU so you should be ok. You just don't have enough plugs.
Exactly. HX 750 would be more than enough for him for a long, LOOONG time.
It's the golden middle: it's not an overkill, but also doesn't cap with his desired setup, has just that exact number of cables he needs and has golden-level efficiency to the boot.
You can't beat that for top-notch CFX setup with such GPUs. It's also future-proof, because it has 6+2 pin cables, and thus he can easily use this PSU if he upgrades to 8-pin GPUs.
It's a PURRFECT PSU for him.