he is applying for a green card
that is not the same as applying for USA citizenship
..................
Having a Green Card (officially known as a Permanent Resident Card (PDF, 1.69 MB) allows you to live and work permanently in the United States.
Yep. It is at least a 5 year wait to become a citizen after you get a green card. I'm not certain that Russia still allows its citizens to hold dual citizenship with the US or that Fedor has ever said he will become a US citizen.