I am doing a 3 tier square cake for a bride and she wants the top tier to be styrofoam so they can keep it forever.  I have never worked with stryofoam before and well, wouldn't the buttercream and fondant eventually rot?    How would I make it so that she can keep it?  When I asked if she wanted a fondant or buttercream cake, she said it didn't matter to her and for me to choose whichever I though would work best.   This is all new territory to me and any tips or advice would be very much appreciated! :)

AI've had dummy cakes for a while (ones I've used for Bridal Shows) the biggest issue I've run into is fading and dust collecting. You might want to ask her how she's planning on storing it-keeping it forever makes me think she wants it on her mantle or something.

I've heard of a product you can use in place of BC, it's made specifically for dummy cakes-hopefully someone will know what I'm talking about and chime in.

Again, I don't know about keeping them forever (for more then a year or so) but I sand them and apply royal then fondant and decorate as usual. They don't smell or anything.

AI don't use icing when doing a dummy tier, so that's going to last a long time. As mentioned it will fade and get dusty. if they keep it in a box and pull it out every year, that would keep a long time. I secretly wish all my customers just wanted me to decorate styrofoam hahaha! :D

AUse royal icing, tell the customer to shellac it

There is a product out there called "PermaIce" it is for display cakes and works just like royal icing.  It is non edible.  I've heard of people keeping fondant figures and toppings from cakes as keepsakes, so I would assume that if you cover a dummy cake in fondant, it would just harden and be fine.  If she wanted to display it there are glass covers and glass display boxes.  

To attach to fondant to a dummy I use piping gel with a brush. THe fondant sticks nicely and nothing rots! Like previous posts already mentioned, it will fade if keot in the light and also collect dust if left out. Other than that- there should be no issues. Fondant won't rot. But dont use BC.