My father in law doesn't wish to be cremated because he fears it will be a problem when the rapture comes...
Comes from the old European belief that if you were being haunted by a deceased person, digging up their remains and burning them would banish the ghost, since there were no remains for it to rest in during the daylight hours.
If they believe their decomposed and liquefied remains will be magically restored, ashes should be no problem.
Well, a few people will have to be reassembled after being food for various land and sea animals; so, I wouldn't think re-hydrating some ashes would be a problem.
Also, I was taught that Christians should not be cremated because it was a statement that you didn't believe in the resurrection of our bodies when the big J returns. My parents later changed their minds and decided their god can figure out how to put them back together. I think this change of belief had something to do with the cost of cremation being cheaper than a burial.
Just say, "ashes to ashes, dust to dust" or just laugh in silence as it won't be your decision. My father could have been buried at the closest vetaran's cemetery free, but my mother nixed that because they bury spouses on top of one another and she feared being on the bottom. He went first by near ten years, but they already had plots by then.
Just remind him that if God can make a man, out of a lump of clay, then re-making the same man from a pile of ashes should be a piece of cake!
Of course, he just made a man from that first lump of clay. Who knows if god can make the same man again as the one from whom the ashes came? The blueprint will be shot by that point ...