I edited the post to be less mean. It's just frustrating because he calls me every day, and usually, when testers tell me about the problems they've encountered, I understand how they happened and what to do to improve the UI.
Not with this person. I literally have no idea how to make the UI more clear.
It's frustrating how often he calls me about matters that are easily solved with some thinking.
I mean problems like "I added a filter to check if a value exists, but it doesn't work", but instead of choosing the "Value exists" filter he used the "Value equals (something)" filter with an empty string for (something). It never crossed his mind to try the other one, even though they're right next to each other in the menu.
(edited to be less mean)
Volpeon 
