I was accused of plagiarism multiple times in high school, since my writing was a bit more sophisticated than HS teachers expect, and so they tended to assume I had copied it out of encyclopedias. Because I had this issue to deal with frequently, I was extremely careful not ever to plagiarise.

That said, I'm also a bit absentminded and I find it very likely that someplace in the body of junk I've written casually is something that could be traced to someone else. Which is the crux of the issue: if you're going to write something for publication, you need to pay way the hell more attention than if you're just hacking away at a story to amuse your friends. So I sympathise with her totally if her work was published without her consent. Otherwise, she was careless at best.