The only icons that should have changed were any PDFs that were saved to his desktop. Are you sure all of them changed?
If so, try this, right click on the desktop and select Personalize. Then select Change Desktop Icons. Select (enable) the "Allow themes to change desktop icons" box. Click OK or Apply. Then, select another theme and let it change the icons (be sure to apply the theme) and then change back to the original theme (if desired).
This should do the trick.
Also, I would suggest he also download, install, update, and conduct a full scan with malwarebytes (www.malwarebytes.org) to make sure he didn't get any nasties along the way of installing Adobe Reader (if he got Adobe Reader from any other site than adobe.com, this is a strong possibility.