I would tell her that sometimes we copy another persons work because we admire it and it is okay to sing or play a song that we heard before for our own enjoyment but we cannot take credit for another persons work. You can make up a scenerio that she could relate to. For example, if she (let's say her name is Mary) had written a really great story,left it at her desk and then her friend came took it and showed it to her teacher. The teacher tells her friend that she did a fantastic job and she could go out and play at recess time . It would fine if the friend said, "oh I didn't do this it's Mary's" so instead Mary got the praise and got to go outside for recess but it would not be fine if the friend accepted the praise and got to go for recess and Mary had to write a whole new story because she lost her work.
