1996 Volkswagen Jetta DIESEL IN AN UNLEADED ENGINE!

Noises problem
1996 Volkswagen Jetta 4 cyl Two Wheel Drive Manual

I have a friend, that had a friend put diesel fuel in his gas tank, FIVE gallons of it. It of course was a mistake, but now his engine kind of sounds funny, and when we were driving today all this cloudy dark smoke came out of the tail pipe. There was oil leaking all the way to the tail pipe. We fixed that problem. The question really is, is did he completely mess up his car? We siphoned the bad gas out, put two gas treatments in, and put 10 gallons of gas in. The car stopped the smoking, but the engine kind of still makes a funny noise. A mechanic that I spoke to said that is could just be the valve springs, because they still might have some of the old gas on it, but I wanted a second opinion. Anything will help. Thanks.