A high quality 550W PSU is more than enough for a system with an i5-3570K and 7870.
http://techreport.com/r.x/geforce-gtx-660/power-load.gif
http://techreport.com/review/23527/review-nvidia-geforce-gtx-660-graphics-card/10
This review shows a system with an i7-3820 (more power hungry than an i5-3570K) and an factory overclocked MSI 7870 drawing 272W at the wall under Skyrim.
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard 500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended
600W power supply (or greater) with four 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology.
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 4GB of system memory
I say let him get the antect psu I picked, he'll have enough juice to get him running and if he decides to oc a little bit.