I don't see a problem at all...I've done multi=layer cakes with no problems....its all one shape so should be fine...you could just make two layers from 3 inch cake pans and split those to have the height from the frosting put between, torting it to get the height....i have used the 12 x 18 pan double layer and that can be split through middle and stacked also...cutting in half and then torting too...good luck...

The rule of thumb is to use separators and boards for every four inches of cake height.

Yes but she doesn't have any other weight on top, its just simple layered cake...not a stacking kind for tiered ...

Still cake can not support unlimited weight, and that has nothing to do with tiers.  I'm uncertain how tall this is going to be.  3 or 4  11x15 layers, if each is 2" tall will be one tall cake!  And obviously would feed more than 40.  Maybe each layer will only be about an 1"?

I know it has nothing to do with tiers...i said that...she said as tall as a beer can so that is easy enough to do....torting it and frosting in between layers can create the height quite well especially if using 3 inch pans to bake...

I make 3 layer cakes often with no support and they work out fine. A cake this size will feed  more than 40 people like leah_s said.