This has been an interesting discussion; no can of worms has been opened. But with all due respect to your friend; I think he or she is mistaken though.

Think about it this way; take an aquarium with water in it and weigh it. Then take a block of Styrofoam and float it in the aquarium with a brick on top of it. The water level will raise equal to the volume of water the Styrofoam block and brick displaces. Finally weigh the aquarium with the water and the Styrofoam/brick. The extra weight of the brick will register, along with the weight of the water unless of course some of it escapes over the top when the brick is placed.

Cheers!
Bill