FCP 6 is over 6 years old. It won't work with many of the current cameras made today. It won't install on the current OS at all, and it takes a hack to get it to install on Mountain Lion (10.8). It is a bad choice, IMHO. Never invest in software that old.
I'd recommend they look at FCX...it's $300. Free demo is available...30 day trial. Also should look at Premiere Pro CC. It's a monthly subscription so it won't break the bank. 30-day trial also available.
G5? Wow. Really getting old old equipment. He won't be able to deal with a lot of the camera formats with that rig. Cannot import footage from an AVCHD camera, for example. The machine lacks the power to convert the footage to ProRes.
Get a Mac mini and X before getting a G5 and FCP 6. I wouldn't even have an expectation of a G5 operating on a daily basis. Things just go bad with computers pushing a decade old. Capacitors go bad, power supplies die etc. Sure he didn't just use G5 as a generic term for the big desktop made by Apple? I've heard and seen many people point to their big Mac Pro and call it a G5.