Both my friend and I train 6 days a week of lifting weights and cardio. More or less we do the same thing and the work rate is similar. The only thing is that I eat healthy and follow a good diet while on the other hand, my friend eats garbage. His diet is basically bread, cereal or fried bacon in the morning and fried food such as fries and chicken nuggets for lunch and dinner. His snacks are chocolate and/or sweets. The only good thing he has in his diet is the low fat milk he drinks + a few fruits. He's most probably never eaten vegetables in his life.
I know I'll be ahead of him eventually but my question is, by how much would it slow his progress down when it comes to fitness? We haven't been training long so would he stop getting gains much sooner than I will?
P.S. I've tried to make and teach my friend to eat healthy or at least healthier but he's almost incapable of eating any other food apart from the food he eats now due to the way he was raised.