Q: Can the NVIDIA GeForce GT 650M handle 4k video editing?

No. The 650M precedes 4K.

I've got to ask, why is he shooting 4K?

It's rhetorical, you don't need to answer, but lots of people shoot 4K for absolutely no valid reason. If he's got a good reason and proper tools to shoot 4K, he must therefore have the tools to edit and deliver 4K. That's a more current Macintosh, for sure, and there are many machines on the used market. Keep looking and he will find something for much less than a new unit (like a production company that found out they can't make any money shooting 4K). Wishing him luck and fun.

MMany people shoot 4K to reframe in post. An interview with two people sitting side by side becomes a three camera shoot.

According to http://www.geforce.com/hardware/notebook-gpus/geforce-gt-650m/specifications the GT 650m should be able to handle 4K at 60 hz using mini displayport. Also, proof here: http://apple.stackexchange.com/questions/136856/can-my-mid-2012-macbook-pro-reti na-15-run-a-4k-panel-at-30hz-over-hdmi.