You seem to be misunderstanding. Read your parent's last line again. "Empty space" is indistinguishable from encrypted data. On the hard disk, everything will just look like randomized bits, empty space and data alike. There is no way to write the program you propose without the encryption key(s). So there's no way to tell, unless you have all the keys.

The program will just overwrite the data of the hidden volume. That's why it's important to have a lot of empty ("empty") space on the primary volume when you have a hidden volume there.