For starters, Dan was awesome. Ran all over the stage, climbed on top of the cabinet of speakers, said "asshole" during the "Call me pathetic, call me what you will" line - you can see Billie Joe laughing or smiling throughout his performance. At the end Billie Joe said "Holy shit. This is the best singer we've had the whole fucking tour", then gave him his guitar.
Dan told MTV that he had planned to take a train back home, but after he got the guitar, Green Day's tour manager said it probably wasn't a good idea, and they arranged for a driver to take him back home. Here's a quote from Dan about joining the band on stage from the MTV article
Here's the video of his performance.
'"There are no words to describe it. If you've ever seen 80,000 people before, that's one thing, but seeing them look back at you, that's something that doesn't even seem real," Michie said. "And then, when I was walking towards [Armstrong] to give him a hug, I was just thinking, 'Oh my God.' You see him in pictures every day, but it's a completely different thing to see him in real life."'
Source : Green Day Authority