I had a friend who was vegetarian for 20 years and even trace amounts of most meat proteins would send her into severe gastric distress.
But I would say that yes, the conversion from PROCESSED foods to real foods can be rough for a lot of people.
I dont know that I would start with nuts, except maybe finely processed nut butters. If she had no problems with dairy, maybe make the conversion to higher fat dairy (plain yogurt) and introduce soft cooked vegetables & meat (pureed soups, etc. ). Make the transition slowly. Investigate making better meal replacement drinks for her with coconut milk and plain protein powders rather than whatever sugar or aspartame stuff they probably gave her.
There also might be room here to look at the more carb laden primal foods so she doesnt toss carb flu on top of gut pain. Mashed sweet potatoes with lots of butter. Small portions of meat when she is ready.
MTA: because it is rare I dont have more to say
"When I got too tired to run anymore I just pretended I wasnt tired and kept running anyway" - my daughter Age 7