The Wilton instructor I saw said if you use water instead of milk in the recipe it won't spoil. I'm not sure it won't melt if outside however. You should find out if they can put the cake in a cool area until 10 when ready to serve.
I've left my buttercream frosted cakes out for 2-3 days with no refrigeration and they have been just great. When I mean "out", I mean, inside your home or reception area. Only if it were going to be outdoors, would I be concerned with bugs and heat. Best of luck to you!
No, it won't spoil. From what I have found out, the sugar in the icing acts as a preservative. If it is left out for a few days it will be fine. I leave all my buttercream cakes out and I've never had any problem.
If you mean on the cake the cake will go bad way before the frosting even made with milk
now if you mean in the bowl that is a different story after a week it starts to get a different taste in the refriderator if you keep the air out I find it still good after a month or more no change in taste and I always use milk.