The division of music into "chords and leads" is largely a creation of rock music, that simply doesn't exist in music in general. Chords are generated from scales and linear movements, if you ever want to do anything more interesting than memorizing fingerings you need to understand your vocabulary.If he"s just starting out, let him play songs and learn chords. When he starts thinking about lead guitar, then you can introduce the scales and modes. But if he wants to stick with rhythm and gets good at it, I think the world might just be better with another solid rhythm guitarist. But he might still want to learn basic scales for that.