Does anyone have advice about resting a styrofoam ball on a cake, please? The cake in question is a 9", 5" high, 3 layer ganache and fondant covered WASC thing filled with cream cheese frosting.

The ball is 6" in diameter and weighs less than 12 oz. Do I need to put a dowel in it and pass the dowel through a tube support in the cake? I plan to have the ball 'bursting' out of the cake by cutting the fondant underneath and folding it back so the base of the ball can be disguised if necessary. I don't need to travel far and can assemble on site if necessary.

Any tips would be great. Thanks!

I'd support it just as a matter of principle. If you're going to be covering the styro ball with fondant or any other decoration, its likely to be more than the 12 ounces you mentioned. I'd run a bubble tea straw as a support, stick a lolly stick (or a skewer) into the ball and run the lolly stick into the bubble tea straw. The benefit of using a skewer rather than a lolly stick is that the pointed end will go into the styro quite easily.

Excellent - thank you. The ball is 12oz with the fondant already on, but I think your plan is a good one. I don't know what a bubble tea straw is though, mind you! I'll Google it.

Actually, an ordinary soda straw would probably do you in this case. Bubble tea straws are the extra large (diameter) straws used for drinking bubble tea. Some people use them as supports in tiered cakes instead of dowels. The idea is that they don't displace the cake and therefore don't contribute to cake shifting the way solid dowels can.