How can I make a copy of an AVCHD disc?

Do you mean from a hard disk camcorder?

If so, connect the camcorder to your computer, and it shows u as an external disk. Use the native file manager (Finder for Mac, Explorer for Windows) to copy the entire folder tree from the camcorder to your computer.

The same works for SD card camcorders. I don't know how things work on a DVD camcorder

I should have been clearer. I burned footage from my HC9 to AVCHD with Nero. Sent it to a friend. He wants to make a copy. Is there a way

IF I understand you sent him a disc with AVCHD on it. He can just use Nero( or any other disc copy software) and make a disc copy. Nero will make an identical copy of the disc that you sent him.
Ron Evans