My friend was driving his 95 Jetta and attempted to take a 90 degree turn in the inside lane. He lost control and wound up in the grass. He popped his tire off the wheel. My question is when did the tire come off? When he started sliding, or when he hit the grass?

I would guess when he hit the grass. Im not an expert, but i would imagine the rim dug into the ground and it took the tire off when it did.

the tire probably blew off the bead of the rim when he took the massive turn and the side wall couldn't handle it. so it gave way. were they low pro tires??
like 225/45/16 or just stock? stock will bolw off easily if the right force is applied.

They weren't low pros. And the wheels were stock.