Darren Fresco said he asked Pistorius immediately after the sunroof shooting if he was "mad" and the world-famous runner "just laughed" at him.
Pistorius denies shooting the gun in the car, although now two witnesses say that he did.
Fresco's testimony in the South African court portrayed Pistorius as a reckless hothead who also asked Fresco to take the blame for a gun being fired accidentally under a table in a busy Johannesburg restaurant.
But Fresco's account appeared to be undermined when Pistorius' defense lawyer created doubts over Fresco's recollection of some events. Fresco responded to a number of questions from Barry Roux on cross-examination by saying he didn't remember. He also said he'd been following some previous testimony in the case on Twitter, which witnesses should not do.
Pistorius, 27, is on trial for murder for the shooting death of girlfriend Reeva Steenkamp on Valentine's Day last year, but also faces two firearm charges for shooting in public and a third firearm charge for illegal possession of ammunition.
Pistorius, the first double-amputee runner to compete at the Olympics, pleaded not guilty to all four charges against him. He specifically denies that he fired the gun in the car, Roux said.
Pistorius's demeanor in court was drastically different Tuesday to the vomiting, retching defendant who needed a bucket to throw up in on Monday as he heard a pathologist give graphic details of the injuries he inflicted on his girlfriend when he shot her multiple times through a toilet door in his home on Feb. 14 last year.