I'd still say just follow /u/drakonka's advice. Just do it in a non-confrontational matter. Simply say this file was included which is legally fine you just need to add documentation attributing the project. You don't even need to point out that she added it. If the files are lifted directly, then there isn't any argument that you're wrong. You can only speculate as to how she'll react. She may simply (honestly) say it was an oversight.

That said I wouldn't worry about the legal aspects. Even if it's unethical, you're company simply isn't going to get in trouble for it. It's not really your problem anyway (though I guess they could be angry with you if they knew that you knew and never said anything).