There is not risk in boiling a dvd, at 100 degrees C the water isn't hot enough to melt the plastic. I've boiled some for over 5 minutes and although it didn't fix them because the scratches were too deep it didn't make them worse. But I no longer think that this is a solution for scratched discs. If a dvd doesn't play but has no surface damage it's probably because the plastic is no longer in contact with the metal at some spots and it screws up the optical reader, boiling them probably fixes that by "softening" the plastic just enough for it to expand a bit and take it's place back.
This has worked for me once or twice but I was never able to fix a scratched dvd that way.
