At a certain point its a safe bet to get a band tattoo. It all depends how long they've been around to some degree. Green Day is one of those bands that's been around and had enough social influence that a tattoo honoring them just makes sense. If a band just came out with their first single, however, you may want to wait on the tattoo. Even if the single is really good. That being said, I've heard of all kinds of band tattoos, but never a recreation of a ticket.
I assume this was the first time they ever saw Green Day live. If not then this seems like an even worse idea. Concert tickets aren't the most well designed pieces of graphic art. There's a lot of information that just has to be there for cataloguing purposes. Did they ever think they could just get 'Green Day' and the date next to it and that would work too? Just a thought.