Question: A technical writer wants to change a label of a table column. He says it is not clear. I, a usability expert, am against it. Who should have the last word?

Answer: 
Ultimately, your instincts should be supported by data extracted from your users. That's not always the case, so why not A/B both of your theories and see which is better received?

I run into this a lot - it is mostly assumptions by one entity on behalf of the user. In my case, it's usually "our users want an iPhone app" - these unqualified statements can be really damaging in the long run. If someone were to say "This is not clear," I'd ask for the opinion of others (or the person saying it to demonstrate that it directly impacts the user). If they feel so strongly about it, they should back it up.

On the flip side, I wouldn't say that just because you're the usability expert, you know that it is right - you are allowed to follow your instincts, but I think it is unfair to ask someone else to produce evidence without you putting in the same effort. Ultimately, I think it is best settled by an A/B with both findings.
I agree that user testing is great, however I have found that there's almost never the time or resources to test UI text formally. The "last word" is typically (for better or worse) the product owner, but all parties should have the opportunity to make their cases.

There's info you haven't provided in the question... such as, WHY does the writer think it's not clear? Is it possible that you're too close to the functionality and are making assumptions about the user's knowledge that the writer can see more objectively? Does the term in question make sense with the other text on the screen? Are there style guidelines that the writer wants to adhere to that the other term breaks?