I've posted about this before. My 2002 LW had a dead battery when I arrived at the Calgary International Airport to pick it up. The lot, run by Budget Rent a Car system, provided me with a boost. The operator assured me that he knew what he was doing, even though we questioned him when he put the cables on the negative battery terminal. His cables were internally damaged as well, and arced a bit as he worked.
The upshot? One burned out airbag control computer -- $ 642.00. One other computer that needed to be "reflashed" - $ 48.00, and $ 100.00 or more in labour charges. Of course, the lot accepts no responsibility, saying that their truck was clearly marked "Boosts at owner's risk" - which it didn't say, although I'm sure it does now.
My advice? Don't let a parking lot boost truck near your car. Stick to the AAA (CAA here). Find a car rental company with better corporate morals than Budget while you are at it. And avoid the "Park and Jet" lots like the plague.
An expensive lesson to be sure.