What are the consequences of a person's involvement in stealing birds, along with refusing to confess to the theft before the court, then attempting to clear oneself of the victim's right, and how is the injustice estimated in this case?

You were obliged to command your friends to repent and return the right to its owner or seek his absolution. If they refused, you were obliged to testify truthfully in court. Do not conceal your testimony, thereby losing the man's right, for whoever conceals testimony and causes a right to be lost by his action is accountable for it.

Therefore, advise your friends to repent and return the right or seek absolution from it. If they do, you are not obligated to return anything to the man. If they do not, then either testify against them or inform the man of the theft and seek his absolution for your concealment of testimony. Generally, merely asking for forgiveness is not sufficient.

And Allah knows best.