Demaestro - 5:48 pm on Oct 5, 2010 (gmt 0)
If it were me without saying why, I would try to implement some type of internal code review sessions. Do it under the premise of standardizing internal programming practices.
When I was at a dev house we would do them once a week. Boss would buy lunch and we would use our lunch hour.
We would take sections of code and review them as a group, pointing out good and bad programming practices, doing theoretical rewrites or optimizations.
Make sure you seperate people's egos from their work. It is not good for your health to become emotionally attached to your code anyways. Once egos are gone it is easy to criticize implementation without hurting people's feelings.
Start with some of your code and then work your way to his, when discussing it as a group you may find there were reasons you hadn't thought of for doing things the way he did. Maybe you learn something, maybe he does... hopefully you all do.
To answer your meta question though.
IMO yes you are allowed to pick holes in your colleges code. In fact if the code is going to produce bugs or security issues it is your duty to pick holes in it.