What is the required expiation, and do I owe DIYYA (blood money) regarding a fight that led to a person's death, knowing that I was not his direct killer?

Your friends erred by supporting you in the fight, and the appropriate action for them is to reconcile between you both, in accordance with the verse: "There is no good in much of their secret counsel except for one who enjoins charity or good or reconciliation between people," and the verse: "And if two factions among the believers should fight, then make settlement between them; but if one of them oppresses the other, then fight against the one that oppresses until it returns to the ordinance of Allah." Also, the saying of the Prophet, peace and blessings be upon him: "Support your brother whether he is an oppressor or oppressed," for supporting him as an oppressor means preventing him from oppression. Your friend stabbing the young man with a knife is a grave crime. If the young man was an aggressor, he should not be killed if it was possible to prevent him. Your friends could have prevented the young man without using a knife. Your friend is the killer, and the guardians of the deceased have the right to retribution (Qisas) from him or to accept blood money (Diyyah). You must repent, and there is no blood money upon you unless you ordered the assault or held the young man. You should consult your heart. The matter can also be referred to religious courts.