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, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
In the case of manslaughter, Diyyah is due on the ‘Aaqilah (i.e. killer's male parental relatives) and the expiation (i.e. freeing a believing slave) is required from the killer himself. There is no disagreement among scholars in this regard, as was said by Ibn Al-Munthir . Scholars unanimously agreed upon this point.
In cases of deliberate killing, the Diyyah is not borne by the ‘Aaqilah but rather it is to be paid by the killer only. The questioner is not obliged to pay the Diyyah in either case, unless he was among the killer's ‘Aaqilah. In this case, he should bear his share of the blood money, if the killing was accidental.
As for your question about giving your friend some money from zakat to pay the Diyyah, if he is obliged to pay the blood money but is in straitened circumstances and is not in a financial position to repay his debt, then it is permissible to give him from zakat money to pay the blood money. In such a case, he is regarded as a debtor in straitened circumstances and is eligible to receive zakat.
Allaah Knows best.