As a full-time employee in the purely technical track, greater responsibility and pay would come through architectural and design roles.

If he really just wants to code, the best way to maximize income would just be develop a career as a consultant rather than a staff engineer. The biggest opportunities for writing code for a ton of money are in a few intense roles, usually associated with R&D on a core product. It's not easy to stumble into those opportunities, and if your friend had the skills to go down that path, he'd already know.