I have a customer who would like a mehndi design for her daughter's 10th birthday. She wants the cake itself to be buttercream. Can i use royal icing to pipe the mehndi design onto the buttercream cake? A fellow cake decorator told me that I can't use royal icing because the buttercream will eat it (i'm using a non-crusting buttercream).

I suggest you test it. I *think* it will work just fine but since I seldom used non crusting recipes I'm not totally sure.