Short of an admission of fraud by the Green Card holder, herself, it would be extremely difficult to prove she didn't enter the marriage in good faith, imo. The couple have already provided a plethora of evidence to the contrary. The fact is that the Green Card holder can continue her path to citizenship by a couple different routes....both without the cooperation of the US spouse. The sooner she becomes a citizen, the sooner the US spouse's I-864 obligation ends.
The divorce settlement is a separate issue, but I think proving fraud would be just as difficult in that context. Good meaning marriages fail every day.