Help... 8 year old humping pillow....

So I baby sit for a living and well my friend asked me to watch her 8 year old niece for the week no big deal. Well during quite time (nap time for the littles) I walked in to my living room to see what she was doing and she was jumping my couch pillow. I just went back in my room... I know it's normal to experiment with the body or whatever but I wasn't expecting it in my house.... Should I just tell my friend and have her talk to her or what do I do from here.... All advice helps!