Assuming your friend already knows javascript, Backbone would be a solid choice. You can add it to an existing codebase as a model layer and then if you really

Assuming your friend already knows javascript, Backbone would be a solid choice. You can add it to an existing codebase as a model layer and then if you really truly need a framework you can tack marionette or chaplin on top of it. If your friend's a total beginner, though, then it's better to just learn vanilla js for now.