I used to be a pretty hardcore raider with a very well known guild on a very high pop server. I raided 6-7 days a week, all night for about 3 years. That pc wouldn't last long in end game, or serious PvP... You'd need something stronger. BUT if he's just playing to play, and has no real aspirations to become the best that will be fine.
When you want to be in a 40 man (now 25 man) raid burning the midnight oil so you can have the bragging rights of taking it down before your competition. Thats when you can't afford even a smidge of lag, thats when you having less then 30 fps can ruin an entire guild's attempt.
Its honestly unbelievable how nice of a PC you need for wow if you're serious. The everyday Joe could get along fine with almost nothing for a PC, but when you raid a lots going on, and your system needs to process all thats going on in a timely manner.
RIP Rakie: Human rogue of <Ethereal Knights> / <Darkfire> and
RIP Rakie: Blood Elf rogue (Reroll) of <Aftermath>
Im so glad I quit, but I really miss playing.