I don't know whether or not this person has truly experienced God's forgiveness -- but I do know this: No sin is too big for God to forgive.
You see, every sin is serious in God's eyes. The reason is because God is absolutely pure and holy, and even one sin -- just one, no matter how minor it might seem to us -- would be enough to banish us from His presence forever. The Bible says of God, "Your eyes are too pure to look on evil; you cannot tolerate wrong" (Habakkuk 1:13). Every sin makes us guilty before God and subject to His judgment.
But the Bible tells us something else about God: He loves us. And because He loves us, He wants us to be with Him in heaven forever. But how is that possible, if we are sinners and He is holy? It's possible for one reason: Jesus Christ came into the world to take away our sins. We deserve to die for our sins -- but He died in our place. Every sin you ever committed was placed on Him, and He took the judgment you deserve.
No matter who we are or what we have done, God offers us forgiveness in Christ -- freely and fully. The Bible says, "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9). Open your heart to Christ today.