Yes your friend can appear for the JEE main ans Advance 2020 exam after giving NIOS examination and passing with 75% or more percentage in the exams. It doesn't matter if your friend passed his board exam in 2nd attempt in 2018, JEE provide 2 attempt to pass the exam so ofcourse your friend can

Hello Vinamra,

As told before, your friend is totally eligible to appear in JEE - Main 2020. You are saying that he has passed his class 12 in 2018 but in the second attempt that won't be an issue. Even if he has done his class 12 from NIOS he is eligible to appear in JEE - Main. He passed his class 2018 and used 2019 for studying for JEE so now he can appear for JEE - Main in 2020.

All The Best.