Short version -- he was Chief Purser and while waiting for the jetway to be positioned, a passenger distracted him. When the CSR knocked on the window, he turned back to the door, forgetting he had not disarmed it, cranked the handle and almost blew the CSR down the ramp.
A replacement for a 777 slide probably runs into the $80,000 or more range. My friend blew a slide on the 757... cost $50,000 back in 1998. A 777 is a double-lane slide and much larger.
The funniest (strangest) part about this is that I think I took that plane out afterwards. I had shown up to my gate at Chicago and went down the jet bridge... when I got to the end, the mechanics told me I had to wait because they had to replace a slide. I looked down and saw the deflated slide hanging from the forward door. "What happened?" "Some guy busted the slide and nearly killed a gate agent." "OMG! Are they okay? Are we cancelled?" "Nope, we'll just swap it out." I watched them unhook the slide from the brackets, drop it down onto the tarmac, bring up a new slide pack, slide it into the door, lock it all up and done.