I think you have some valid concerns, especially if this is code you're working in tandem with.

I think you need to take the human approach here and just ask the person that wrote it or is maintaining it.

"Hey $author, I was taking a look at this code you wrote and I was curious why you did $thing this way?"

This way, it's not confrontational, and they don't really have an excuse to get defensive. You're there purely for educational purposes. I've found a lot of times people get kind of a sheepish look and go "yeaaaa it's really bad code, sorry, I was in an enormous rush etc." If they try to justify it with something stupid, well, you have your answer then.