The fondant dries out over time, just like gum paste. So while it is technically made from an edible material (which satisfies the local health dept), no one said that it would taste good Usually I tell clients that the decorations can be removed to make it easier to cut the cake if they are complex, or if there is a lot of fondant on the cake (think about the harlequin tiles on a layer of fondant) so you might want to suggest this to your friend if you are not going to be there when the cake is served.
Things on wires I make ahead of time, at least a week if I am organized, the day before if I am not If I need something to stand up (say, a monogram), I use gum paste and make several of them, and make them at least 10 days prior. I’ve found that some brands of gum paste dry faster (overnight) than others. Mostly I buy from Albert Uster Imports. They just reformulated their gum paste and now it is fantastic. Expensive, but I can make bow loops before I leave for the day and they are bone dry/rock hard when I arrive in the morning.
Flat decorations (flowers, dots, etc) I make when I am finishing the cake.