What to know
The perceived “excessiveness” of the Christian message is being questioned here. Many Muslims view the Gospel as garish; unnecessary both in scope and claim. Others see this as a belief that the Gospel-view of God limits him. How could God need anything to forgive?
What to say
God showed that he has the power to substitute one life for another in the story of Adha (also called Qurbani). When Ibrahim was asked to sacrifice his son, he was given a ram instead. In the same way, God chose to substitute another life for ours.
Ask your friend what kind of an animal, or how many animals, would be equal in value and dignity for a human's sin. What about for all of humanity's sin? We believe that God alone could bear the weight of that punishment. He chose to take human form in order to do what no animal (or series of animals) could do. The Holy Book doesn't refer to this as weakness or defeat: This is called "the power and wisdom of God."
He is powerful enough to take human form and still be God.
He is powerful enough to exist in more than one form at once (as God in Heaven who poured out punishment, and as the body that received it.)
He is powerful enough to conquer all sin, death, and Satan (Shaytan)!
He is powerful enough to rescue even the worst of sinners. Read 1 Timothy 1:15 together.
Your Muslim friend is likely to agree that God can do anything unless it goes against his character. The Bible teaches that God gave out the appropriate punishment for our sins, and God absorbed that punishment. God alone has the power and authority to do this.
Read Romans 3:19-31 together and discuss.