I've been painting bikes for years, and this is not so much the painter's fault as the gas itself being the culprit. There's just something about today's gas that just doesn't jive with today's modern paints. It took the very same thing happening to a couple of my own tanks to learn. Most catalyst hardened polyurethanes will take gas being spilled or splashed on it. But if it gets underneath it, yer screwed, especially at the filler neck. I now always leave the paint line on the neck as far down as I can. Brushing a thin coat of 2-part epoxy over the paint line seals it off nicely. You have to seal that line. You can usually save it the second you see it starting, by cutting around the affected area with an Xacto knife getting back to solid paint again, then applying the epoxy. Or better yet, as mentioned, hopefully keep it from happening in the first place. Breaks my painter's heart to see your tank, but everything can be fixed. If he can repaint the orange above the stripes, sand out the drips down the sides, and then re-clear the whole tank that would be cool. If not, just start over and chalk it up to experience.