I agree the forward to a friend feature is extremely useful! We have added the feature within the Cross Device Editor.
The Cross Device Editor an editor that allows you to edit a campaign on a mobile device or tablet or on desktop computer. It has several features from our existing Third Generation Editor and many features you find only within the Cross Device Editor/Templates.
To find the Forward to a Friend option, you do have start with a template that uses the Cross Device Editor experience. Here's how!
Then within the Cross Device Editor template, here is how you add the Forward to a Friend button.