Ernie...Shoe is correct that you need the external HD adaptor and I'm assuming your customer already has this; if not it is needed. If so try this:

1.) Press MENU-1-3-7-0
2.) Use the up and down arrow keys to select DTV INPUT. The default selection should be AUTO
3.) Press ENTER until the default select switches to HD.
4.) Press MENU twice to exit the service menu.
5.) Now you can use the INPUT button on your remote to cycle through the inputs; there will be a new input immediately following ANT-B; this is the HD input.
6.) TV is ready for HD