So let me get this straight, the creationist are arguing that because the industrial revolution was man-made it’s unnatural and makes the peppered moth evolution a product of mans meddling?
If that is the or a argument they are using here, I'd reply;
It doesn’t matter if we stimulated the mutation or not, all that matters is something affected the moths. It stands to reason that natural events have occurred through out the history of the earth that has no doubt created effects of one kind or another. Volcanic Ash, Temperature change, geography (plates moving or animals being swept to different location). To use the effects of something man-made holds just as much validity in pointing out an effect of evolution, as long as the man-made effects where not purposely to stimulate evolution. There is nothing magical about a man-made effect that takes them out of the category of environmental effects, there is nothing that makes man made effects unsuitable for examples of effects that can cause evolution.
I don’t know if this is what they where arguing, but regardless I come across this in such debates often. I would compare this to say; trying to discover if a person bleeds naturally when the skin is breeched. Do we really need to see a person accidentally get a cut in the wild to see if bleeding occurs naturally? Do we not gain information about the nature of bleeding from man-made cuts? It would be stupid to say bleeding does not ever occur because I have never seen it occur naturally and if somebody purposely causes bleeding it doesn’t count. If we do something and the effect induces evolution in a animal or ourselves it tells life evolves (and we can witness much of this in adaptation, why is evolution excluded?).
Edited by Akrid, : Added some "?" marks and stuff 
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