Why did my buddys 01 KX 250 jus die out?

He jus bought this bike 01 kx 250 from a guy who had raced n jumped it n it had been rebuilt recently new tires chain u know anyway i was riden my 96 kx 250 and he was on his this was 2 days after he bought it and he hadent really riden it til now and ok we are rollin down the road in 5th and his bike jus died and it was hard to start n wen we did start it, it sounded terrible like rattlyin in stuff plus wen we were tryin to start it once it threw out a big puff of white smoke. I told him to call the dude up but he wouldent or he wouldent let me so any help wo0uld be appreciated oh yeah it cost $1800 Thanks curtis