The following should accomplish what you want, but first I want to state that this is generally not recommended. Letting someone have a copy of your private key allows them to both decrypt anything encrypted to you, and also to impersonate you by their then being able to sign files and email with your private key.
In PGP Desktop's All Keys, you can right click on your key and select Export. Make sure that you make the selection to include your private key in the export. Your assistant can double click on the exported key to import it to her computer. She should then go to PGP Desktop's All Keys, right click on it, select Key Properties, and set Trust to Implicit.