All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Since this person has smoked the weed of his own free will and died because of that intoxicating weed, then there is no liability on the owner of the weed and there is no expiation due on him, but he is sinful for smoking this weed and helping his friend who died due to smoking it. So, he should seek the forgiveness of Allaah for smoking that intoxicant and sincerely repent to Allaah.
We have already issued Fataawa 86558 and 84951 clarifying that all kinds of intoxicants are forbidden, so please refer to them. Also, for more benefit on the conditions of sincere repentance, please refer to Fatwa 86527.
Allaah Knows best.