Here is a check list to review before you let the client have a go.

That moment in time when you suggest to a client that they will be able to add their own content. It's quite straight forward you say, all you need to know is how to Copy & Paste. 

What else am I presuming? That the system has been set up with a WYSIWYG editor and it has been both enabled and configured. You know WYSIWYG is enabled when the familiar toolbar is at the top of the edit box. Oh, and yes I think it's safe to presume that your client is somewhat familiar with at least one word processing system.

A typical WYSIWYG Toolbar: 

What is there to explain?