I'm actually still not quite sure what you can use infiltration for besides sneaking past someone? I mean if I want to get into a building without being seen do i just roll infiltration? once i'm inside do I just keep rolling it as i move about the building? Take into account 9 hit's is WAAAAAY crazy. Means your a stealthy ninja if you take the "Difficulty Table" into account. I try to go by that table as much as possible.
I believe the table went like this...
1 Hit = Easy
2 hit = Moderate
3 hits = hard
5+ hits = Extreme
So considering 5 is extreme, nigh impossible things are probably doable with 9 hits. You do have to remember however what makes sense. The GM trumps the rules every time if something doesn't make sense or if he wants it to slide to let the play feel like they are awesome. Most of the time if the PC's get 5 hit or more i'll let them have whatever they are doing threshhold wise.

Especially if they did an opposed test and one by that much.
I mean these sentries, how do we know he wasn't bored on watch and watching a video on AR because, "Nothing ever happens on our watch." Maybe he was picking his nose and distracted, I don't know. Hell I know when i'm reading things I've been known to totally ignore a conversation when the person is standing right next to me talking to me for about 3 minutes and i look up. "What? Were you talking to me? How long were you talking?" When reading something interesting the outside world for me goes away.
Also consider are these sentries the important part of this mission? Is it meant to be a challenge or is the challenge waiting up ahead? You also can remember that the more people observing or something the more dice people get. I actually think for every additional person you get like a +1 dice.
Finally, you could always throw an adept at him with Enhanced perception 6 gives you like +6 dice, and have his intiution high, and perception high. If he still wins, then damn he is that good. You could also have them use Edge during the perception test.
Edit: noticed Arc's post. Yeah if it's totally being abused than don't let him get away with it but don't take it away from him either. For example maybe there's a camera watching where the guards are... and if the guard's suddenly "disappear" from the camera than security will tighten.