:evil: If you haven't taken the car back to the place where you had the gasket job done, then you should do it now and let them know that the noise started happening after the gaskets were changed. It's probably something they did. If the noise was IMMEDIATELY after the job was done then they should find the trouble and fix it for you free of charge.
I still don't know if that v6 is an OHV. And are you sure that it's backfiring. If its sputtering then it might be because the valve opening times are slightly off but I don't think it would be causing that loud noise. Also you don't know if the oil pump stopped working and your friend continuied to drive the car. She could have damaged the heads but this is only a possiblity! I'm not saying that this is the problem. The reason I asked you how aggressive she drives the car is because I also I had something similar happen to me with my dads gm. Since I hated the car (its a gm) I raced that weak 4cyl. engine every single day, religously, to and from school. I cruised at 90mph+ and it was a 21 mile trip one way. A month later a loud knocking sound could be heard and it got VERY LOUD as I drove it more and when I accelerated. Turns out that first the oil pump went and the #1 pistons ring was cracked?