What is a concrete proof of fault then?
Photos of him out with his mistress. Witness testimony. Texts or letters between the two of them.

I understand that fault is involved. But sometimes, it makes more sense, costs less money, and takes less time to just go for the no-fault divorce. You don't really want to have all of your personal business made a part of the public record just to punish your philandering husband, do you?