So all these Pull apart cupcake cakes are usually decorated with FBCT, but i got an order to make a pull apart cupcake cake piano. the women asked to make 30 cupcakes, 3 by ten. cover them with butterceam and then on top put a large piece of rectangular fondant to cover all the cupcakes and make the piano on the fondant

this kind of beats the whole point of a pull apart cupcake cake right? you wont be able to pull the cupcakkes apart with fondant on top, i explained that to her but she insisted she wanted it like that

My only worry/ question is will the butterceam drop of the cupcakkes or sink if i put fondant on top of them?

The buttercream will be pressed down to a certain extent, but it won't fall off the cupcakes. It does ruin the point of the tear-apart concept, though. As long as she knows she'll have to cut them all apart or take the fondant off in a big messy sheet before serving it (which WILL rip the icing off the cupcakes) then do it. She'll learn the hard way. Just tell her that it's going to defeat the purpose and be a huge mess and let her decide if she wants to do it or not.

Have her sign a disclaimer that you do not recommend it and it will react just as costumeczar explained above. Then do it, and take her $$$$.