At fifteen I would think a person would know that this is a crime, and just overall isn't the right thing to do...when you discover human remains, you have to notify the authorities. One, it is possibly a crime scene, and she is tampering with the evidence (in fact, disposing of it), and two, even if it isn't a crime scene, it is not her right or responsibility to be the one doing the burying--she may feel it is the respectful thing to do, but that person may have had a family somewhere who still doesn't know what happened to them. The family of the deceased person would want to have the body identified and be able to hold their own burial, according to their own traditions.
If someone dies with you while out in the wildnerness, and you can't take the body with you, I think that's a different situation--you can't leave the body out in the open where it will be eaten by animals. But that's not an issue anymore with the skeleton.