His friend became an atheist then he died. Can he offer the funeral prayer for him and say du’aa’ for him?

No File Exists
Check below answers in case you are looking for other related questions:
Following the funeral procession of a kaafir
Following the funeral processions of those who worshipped graves

No File Exists
Related Answers:
Ruling on one who dies and it is not known whether he was a Muslim or a kaafir
Is a person who dies as the result of being struck by a car regarded as a martyr?
A woman who has been divorced by a revocable talaaq may be washed by her husband
Following the funeral procession of a kaafir
Following the funeral processions of those who worshipped graves

Recommended answers for you:
Disturbed by Waswas (Whispers From the Shaytan) and Evil Thoughts
Does women’s hijab have to be black?
Her husband treats her kindly but he does not give her her shar‘i right to intimacy; what should she do?
His wife is not very interested in intercourse so he resorts to masturbation
Reading Soorat al-Mulk protects one from the torment of the grave