I would have them try both and see what they like more. I came from a backend development background and found the most difficult thing about frontend stuff is

I would have them try both and see what they like more. I came from a backend development background and found the most difficult thing about frontend stuff is figuring out what the best tools, frameworks, etc. are to use. As soon as you learn something new and shiny like Angular, Bootstrap, etc. there's a new thing like React and Flux to check out. The build systems for frontend tools are IMHO a pretty big mess, annoying to use, and changing very quickly too. Regardless of which path you take learning solid algorithm and software engineering skills will help with any development you do, frontend or backend. Consider your friend might not even care about web stuff too, perhaps they'd rather make games or mobile applications.