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: 
Per Chad's comment, the product owner is the arbiter of such decisions. It is up to you and the technical writer to make your case for the change; vice versa it is up to the product owner to gather the evidence from you two and any other sources.
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?