Billy Graham is what the Bible would call a "false prophet." Why? Because he taught a false gospel. Billy told people to pray a prayer to receive salvation. The Bible tells us to just believe -- to place our trust in the one and only Savior, Jesus Christ.
Jesus, God incarnate in human form (just as he took on the form of a pillar of fire, a cloud, a rock, and Melkizedek), took on the sins of the world on the cross. He was paying the debt we all owe because of our sins. We are all guilty; we've all sinned; we all deserve hell. But Jesus loves every single one of us so much that he went to hell in our place! He loves you; he doesn't want you to go to hell. He went there for you.
If we try to get to heaven by the merit of any good deed, we nullify grace. "Grace" means "to receive what you don't deserve." If I try to receive heaven based on some deed that I have done, I'm trying to deserve heaven. That would be to nullify grace.
Every person who prays the "sinner's prayer" to receive the free gift of heaven is trying to earn salvation by that good deed. This is why Billy Graham was a false prophet. He led hundreds of thousands to have a temporary false assurance of salvation because they prayed a prayer.
Evangelists should preach in the open air. However, they should come across as rational and normal. Trying to share God's love: trying to share that He suffered to save you.