It depends how much time he spends per day, no? I always thought of Duolingo as something you do in 15 minutes of downtime here or there. Nowhere near the level of time commitment that a class would be.
Hrm maybe you're right. I was doing 50 lessons a day, spent thousands of hours on the app. Quit my job even. Maybe I just suck and am a horrible person. Thanks for bringing this to my attention.