It couldn't have gotten to Diego Garcia, though I suppose it could have gotten well within the range of a drone or fighter so that's irrelevant (I believe even on its original course it would have been in range).
However, here's the thing: hacking does not work like this.
First, a hacker can't just take remote control of a plane like that. Theoretically, you might be able to compromise the computer (if that thing is internet-facing I just want to reach through the internet and punch whoever thought that was a good design idea), but that doesn't give you control of flight systems on any commercial plane I know of (again, that's kind of a glaring design fault and not just a security failing if it can happen!). The best I think you could hope for is crashing the computer and potentially the plane.
Even if it did, rule 1 of computer security: Physical access is the trump card. It will be very obvious very quickly if the plane is obeying some outside inputs, at which point you pull damn the network plug and take command yourself. Your passengers might be pissed that they can't Tweet pictures of the obnoxious passenger in front of them, but they'll live, and more importantly, they won't die. The radio and other tracking/identification/communication systems don't work over any form of TCP/IP that I'm aware of.
This guy's watched too many awful Hollywood hacking movies.
Hollywood version: Hacker furiously typing away trying to get through the third firewall
Real version: Script running while the hacker is making a sandwitch
Hollywood version: Counter-hacker furiously typing away trying to keep the hacker from getting through the third firewall
Real version: IT guy unplugs the box. If it's something that important he notifies whoever's in charge of the other multicast sites that his node will be offline for the moment.
Hollywood version: Hacker using six computers scrolling Matrix text to break a bunch of encrypted passwords
Real version: Hacker calls the company he got the database from and says, "Yeah, this is Mitch at the San Diego office. We need to verify the key today..... Wait, was that M as in Mary or N as in.. uh... Ok, Mary, thanks." *click* (Stuff like this works distressingly often)