In order for your clients to download their favorites, make sure you've done the following first:
Step 1: Enable the Right Download Settings
Make sure you have enabled your gallery downloads, and the All Images switch is on in the Download Type and Size section.
If you want to allow clients to download their favorites only (not the whole gallery or other folders):
Go to the Settings By Folder section.
Click on the Selected Folders radio button, then turn the Favorites switch toggle on. Make sure the other options are turned off.
Once you have finished changing your download settings, click on the Save Settings button to save your changes.
Step 2: Go to Your Gallery
Once gallery downloads have been enabled and a favorites folder has been created, the name of the favorites folder will appear in the gallery menu.
In the menu of the gallery, click on the Favorites folder, then click on the Download icon.
A Download Photos box will prompt you to select the folder(s) you want to download. Click the Favorites checkbox, enter your email address, and click the Next button.
Once you see the confirmation message, an email will arrive in your inbox with a download link to your favorites.