It is sad, that instead of seeing his mother’s funeral as an opportunity to reconcile with his family, by his absence, your friend will assure that the hurts in himself and in his family, grow deeper.
To not be there at his mothers’ funeral sends a seemingly heartless message to his siblings and family…however God knows everything concerned.
If he fears being present perhaps he could send condolences to the family, whom his mother loved, as indeed she loved him? This could give him some relief, that he did a kind thing whether appreciated or not, an act that might honor his mother…as a mother wishes compassion, unity and love in her family
It is never good to put fear before love, and this appears, at least, to be what your friend is doing. Love is what is required of us. But I will not say whether or not it is sin. That is between God and your friend and not for me to judge. Even if I knew the circumstances of his alienation from his family, (which I don’t) it’s still not my place to judge.
May God bless and comfort your friend, and may God bless and comfort his family,
and may God have mercy on his mother’s soul.