I, too, have a mage, and it's my main, so I spend a lot of time on it and thinking about how to do certain things. I prided myself on not being cookie-cutter, I loved being mildly competitive in the Naxx25 pugs I did. After dual-specs came in, I was able to make two specific specs: one for raiding, one for instance grinding, where before I have to make a spec that wasn't optimal for either. After spending maybe 300-400g on just the raiding spec, I decided, just for giggles, to see how the unoriginal, cookie-cutter specs measured up... The findings left me feeling kind of empty. It turned out I only had a few talents off from the cookie-cutter 57/3/11 Arcane spec, and again, having been so proud of how well I was doing in my very own spec, it hurt to see that it wasn't far off from the cookie-cutter at all.
My own Arcane raiding spec.
Pretty much, the only thing that differs is that I have full points in Prismatic Cloak, this is because I find a few situations where I need to drop aggro from adds or the boss, the two points in Student of the mind would only give me 18 MP5, and Slow is never used in raids (if so, not enough that it's worth keeping, unless you can give me a good example of how it can be used effectively), I find.
The only thing I can say is maybe the mage the OP talked about honestly DID think they had made a good spec of their own? They may not know their class as well as I know mine, let alone as much as you know your mage, but it's still possible. If they are anything like me, they'd be happy to share the exact reasoning behind every point they spend in their tree, so it may help you find out if they really DID make that spec themselves and how well they know their class... You don't need the spec the experts say you should use to prove you know your class, you know your class when you can reasonably explain the choices you make with your trees and use them well.