For a Small Sampus design with less than 100 or so end stations (including all kinds of edge networking devices)it won't hurt to collapse the core and distribution layer into 1 layer like what you customer is doing. But obviosly this limits future growth and scalability of the model. You need a solid core and distribution layer in Multilayer model so you can grow bulding blocks, which need aggregation into the upper layer. A Small Campus design restricts that. Also keep in mind where you use fixed configuration switches as opposed to modular switches, again for scalability, future growth and performance, modular switches tend to have a longer life cycle.
