I have a client wanting his cake. I have never done anything like this, help!!!!! 

the décor? the mountains are bc icing applied with a spatula -- then the trees are just piped over -- think about just dropping a blob of icing on the top of a cake and taking the icing spatula and moving it around to thin it out and spread it -- that's all those mountains are -- then a little piping that you can smush down with an uber clean folded towel artfully placed to smush out the piping lines --

then the trees are just piped and overpiped lines 

it's easy -- practice on something -- you got this

Yes, I agree.  Just use a small off-set spatula to spread icing around then highlight it w/piping.