Unknown reason as to why he read it before hand. He has extremely limited GMing experience to my knowledge so I doubt it was for GM prep.
Well, in this instance, it's reasonable to assume he simply wanted foreknowledge of the scenario. Perhaps he wanted to see if it "fit" his character - who knows. But it's cheating, flat-out.
That said, PFS obviously does allow people with foreknowledge to play the game - namely, those who've GMed it or (in less common circumstances) will be GMing it soon, and just had to read it already (maybe they're running it tomorrow). Now, there are GM-star replayers as well. The Guide is pretty strict that these players have to play "dumb".
I'd prefer the person simply not play at my table - I loathe a cheat - but if somehow that wasn't an option, I'd completely forbid any kind of in-scenario prep (no purchases), disallow spell selections and other preparations which implied a knowledge of the scenario, and simply block any attempt to use knowledge of the scenario ("nope, there's no secret door in this room" "No, the cleric has no wands on him - why do you ask?") If this inconveniences the other players, I'd make sure they understood that it was a consequence of someone having read the scenario.
Cheating needs to be shameful. It really needs to be harshly punished, but the nature of PFS makes that challenging (it's hard to really ban someone from play, for example). I've seen Organized Play campaign in which it became "the culture" (Living Forgotten Realms) to seek out scenarios with favorable items and so on based on foreknowledge, and that's a very slippery place. LFR was so bad that people would publish spoiler lists of scenarios with listed items and boons so that folks could plan their farms (oops, I mean play schedule) accordingly.
I certainly hope this new player at least felt stupid, if not embarrassed.