Leviticus 5: God made confession part of the Law. Man was commanded to bring a Lamb as a sin offering to a priest, confessing his sin.
Matthew 5: Jesus did not come to abolish the law, but to fulfill it.
John 1:29: Jesus is the Lamb of God. An offering from God for our sin. Now we only need to confess our sins, and no longer have to bring the sacrifice since Jesus fulfilled the Law and became the sacrifice for our sin.
Matthew 9:1-7: God has given authority to man to forgive sins through Jesus Christ.
James 5:16: We are commanded to confess our sins to each other, and to pray for one another.
John 20:23: Jesus breathes on them and gives them the power to forgive sins. The only other time God breathed on man is when he breathed a soul into him (Genesis 2:7). The next time will be when he destroys the Antichrist…the man of Lawlessness (2 Thessalonians 2:8).
That should at least get you going explaining it from a biblical perspective. I’d suggest praying about it prior to trying to explain it to this person.