I've got a truly crazy level of food sensitivities and my mother was just about as badly off--and that makes me think of a slightly different theory that has worried me and I suspect applies to your wife:
Rather than rejecting nutrition in general the scenario that concerns me is reacting badly to something essential. All it would take is one component, not all nutrition.
To date I have reliable sources of carbohydrates and amino acids, but fatty acids are another matter. I have never found a source of highly purified fatty acids and as far as I can tell not even the TPN formulas are made that way.
I don't have gastroparesis issues but while AFIAK it was never labeled that my mother would react that way to eating something wrong. I can easily picture her getting sick from TPN containing something she was sensitive to.
It's not something that would have been of much priority for the doctors to test--if you can't tolerate all of the ingredients there's no value to knowing you can tolerate some. As far as I can tell your survival time eating no fat is a lot less than your survival time not eating, period. (If you're not eating your body draws on fat stores, thus providing fat. If you're eating but no fat the body doesn't draw on fat stores and bad things happen.)
