I just received an order for a b-day cake for Monday, problem is that I am going away this weekend and won't be back until late Monday. The customer (friend o'mine) wants the cake for tomorrow, which she will keep until the party. Will the cake be alright? It's buttercream icing, should it be kept in the fridge the whole time or would it be alright to leave it out? Help......

With buttercream coating, the cake should be fine if you keep in a covered box. I've read where refrigerating a cake will actually speed up process of drying a cake out. If you wanted, you could always brush a light coating of simple sugar over layers before you fill / cover with BC. (Simple syrup is equal mixture of sugar and water, boiled on stove, then brought to room temp then lightly brushed - not soaked - on cake to add moisture. You can even add a few drops of extract for added flavor).
HTH!!
~ Jaime

If the customer is willing to take a chance on freshness then I say go for it.
I have kept cake, covered in plastic wrap, in the frig for up to a week (not for a customer - for family) and we thought it was just as good at the end as it was the 1st day.