in reply to
How do you critique another person's code?
Code reviews are hard. That's because it's really hard to take someone else picking at your code the first few times it happens. I've had a hard time accepting criticisms in code reviews, and I know how important it is to have them and to listen.
My advice is to think about how you would feel on the other side of the table and act accordingly. Be as nice as you can about it, or the guy will reject everything you say out of spite.
It's also a good idea for your company to invest some money into training resources for this guy. Maybe a good book about style and maintenance issues, like Code Complete. Maybe let him ride a more competent coder's coattails for a bit, doing maintenance on some solid code. I learned a lot that way.