Ok, so let's see if I have this straight:
Your friend and his girlfriend are out water-skiing. (Off-topic: where was the third person? It takes three people to water-ski safely: one to drive the boat, one to ski, one to watch the skier.) At some point they do something to tick off the other guy.
Your friend and his girlfriend drive the boat back to the boat ramp or marina, with the other guy in pursuit. At that point they either need to put the boat in a boatslip, or they need to get it on a trailer.
While the truck was backed up on the boatramp with the trailer in the water, the other guy shows up wanting to kick the snot out of your friend. At that point they probably can't just drive off - the boat may or may not be secured properly to the trailer. So, they're stuck.
Your friend happens to have his pistol in the truck. While the other guy is assaulting your friend's girlfriend, he grabs the pistol and dials 911. Your friend warns off the other guy verbally three times; other guy keeps agressing. Your friend points the pistol at the other guy; the other guy backs off.
The rest is history.
Is this correct?