Also, generally managing tone - "Looks good! Don't forget to add the doc comments on functions X Y and Z" vs "Why are X Y and Z missing doc comments?". Both have the same fundamental information (X Y and Z could use doc comments), but:
With the former, you're acknowledging progress, a "good" first draft, and pointing out what remains to be done in an ego friendly way ("I know you're good enough to make this even better") that doesn't demand a response.
Meanwhile, the latter demands a response, all of them negative. Either "it doesn't need them", or "I forgot them (because I suck)", or silently ignoring the question. None of these are ego friendly.
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If the code just plain sucks, however... I'm not really sure what to do.
And if the coworker gets defensive even with positively slanted feedback... maybe there needs to be a conversation with them, to try and shift their mindset. Show them you get things pointed out in your own code reviews, that it's a team game where everyone is just looking out for each other. That bad code doesn't mean they're a bad programmer, that critique of their code isn't a critique of their skill, that everyone gets blind to the problems in their own code and benefits from a second set of eyes.
Maybe get them in your shoes - helping critique someone else's code, and help explain that they're not being an asshole when they point out edge cases and ugly code, but that they're being an asshole to future them when they have to go back and add a feature, fix a bug, or handle a corner case they'd missed.