In other words and a bit more serious: why wouldn't you work for Google? (or other major player for that matter: Yahoo!, Microsoft, Amazon, Facebook...) First, I thought "what was going on his mind?" but then started thinking deeper about this. What's your opinion?
Google might be a lot more dev-friendly than most big companies, but it's still a big company. Even at Google, that means you have to deal with bureaucracy, corporate politics, etc. It also means that the things you're doing might be shut down by management or just neglected. There's not nearly as much choice in terms of technologies you're using. Google also doesn't pay amazingly well either from what I've heard.
And if you've managed to get an offer from Google, you probably have other options, too. Picking Google just because they're Google seems a bit short-sighted to me. There are very possibly other opportunities that are a better fit for you or that you might like more.
The decision about where to work is very personal, and it includes a lot of factors that are hard enough to sort through when it's your own life and you have full information. Not everyone's going to be drawn to the most lucrative or most prestigious position.
Some people may not want to be a minor influence on a large machine. Perhaps he didn't think his position there would have any measurable (or notable) impact and that wasn't what he wanted.
Alternatively, ask him. He's the only one that knows the answer.
A job with Google, Microsoft, etc probably looks better than average on your resume when you're job hunting again in the future, but other than that I'm not sure why you'd evaluate it any differently than you would an offer from any other company.
I could think of a million reasons not to want to work at Google, or any other major player, ranging from having to relocate through having a problem justifying working for a multi-national, huge company, to simply not liking the name.
Or all of the above.
Personally, I'd shake his hands for having the strength to say no.
* family / friends / girlfriend
* leaving his home country
* having a nicer job
* not being tempted by the challenge
* not being tempted by the money
* being scared of the big jump
Also i've heard from the same girl that Google sucks hiring cool people, like a friend of she applied for Google and they contacted him 6 months later that he moved to SF to work for another company, from a 1 year of interviews. 4 years they called him again just to give feedback of the process.
Or, maybe they didn't pay enough.