There is only one gospel and I believe that Billy preaches it. But even supposing that he wasn't, have you heard the story of the heathen man who heard someone preaching to a crowd? This man, to impress his friends and to ridicule Christians decided to get his own soapbox and start "preaching". Someone was actually saved from his mock preaching. Though the speaker was an unbeliever, the gospel went forth and someone was saved.
I contend that the arminian gospel is a false gospel. Do you disagree? I am unaware of the Bible teaching us to preach a false gospel to save anybody. The truth that God uses evil to His good does not excuse wrong. Do you have any verification of your story? How about something of evidence that Billy Graham has been "astounding"?
