What does “one thing” mean here? Is it an expression?
The expression to focus on here is actually "in her favor," which means to have something to be to your advantage or benefit. Thus, "that's one thing in her favor" means that there is at least one "thing," item, or advantage that she enjoys. Ex. She's smart and talented. She has many things in her favor. Ex. I exchanged some foreign currency yesterday. Thankfully, the dollar is rising in my favor.