Which is more fun may depend on the person's skills. For instance, a person with a background in design may prefer front-end. IMO, it's best to start the person off with at least basic understanding of HTML, CSS, JS the reason being that all web sites have a frontend, but not all a backend. Codecademy Web Fundamentals can be good for this. I guess in the end, really depends on what kind of web dev the person wants to do. The person will be motivated to work on what he or she wants to build for pleasure.