An end result of 10 against a rolling keg? Sounds like a sure hit!
As the others said, 10+1-5+2 (base + size - dex 0 + circumstances) sounds about right. Definetly not more than +2, maybe even less. Yes, the barrels are moving, but I don't think their movement was very fast or erratic. Barrels rolling down the street have the tendency not to dodge your shots.
Even if it didn't quite it, I'd be inclined to make it happen because the players thought instead of charging blindly at the enemy.
It would probably not have been an explosion, but the undead would have had barrels of burning liquid to deal with, with all that lighting up like a christmas tree going on.
Anyway, as the others said: Hang on and see what happens. Maybe it was a one-time thing, just to get it out of his system
I did it myself, occasionally: Have my pet NPC show up and blow things up big time. Guilty as charged. I didn't do it encounter after encounter, though, just once or twice, to show the party that there are powerful bastards around, the world being a dangerous place and all (Incidently, it was an evil campaign, he sent them on their mission, and was my in-game reminder that stupid infighting would not be tolerated. I did that because I had seen evil games befoe where the party was constantly backstabbing each other, players using meta-game knowledge to "win" and all that crap. Sickened me.) The rest of the time (which was easily more than 99% of the time), the characters were the stars.