21.5 - 1 (365 days) = 20.5 year old.
It seems this kid is OK. So, if the I-485 denial reason is due to the out age, your friend can file an I-485 Motion to Re-open/Reconsideration to justify it. Your friend had better find a good lawyer to deal with this issue. Anyway, just wait for the Denial Notice and find the denial reason.