This is your best option. Code is usually hard to read, and anything you can do to alleviate that is good! If you focus on what you think would be a positive outcome of your advice (without phrasing it to emphasize the current negatives, i.e. that their code is unreadable), then your advice will be less embarrassing to hear and more likely to be followed."If this were in a function, the code would be easier to read."
I've worked with dogmatists and I ignored their advice for a long time because I didn't see the value of it in the code they produced. Then I read the original source material of their dogma, saw that it was good, and saw how wrongly they were applying it. Then I became a better programmer and quit. THE MORAL OF THE STORY IS: Make sure I understand an idea before I dismiss it because someone I don't like misunderstands the idea themselves. This is a variant of strawman fallacy.