@Flumps, I understand you don't want to share your partner's diagnosis. Am not speculating and am only posting it as it may help others and may be far away from your situation.
My son was hospitalised a couple of months due to high temperature and severe abdominal pain. His diagnosis was a common bowel condition, diverticulitis.
He was just given the advice to eat more fibre. When he got home after whatever he had to eat he was in agony. We checked Dr Google and he went on a clear fluid diet for a few days (just water, apple juice and clear broth). After that added scrambled eggs, custard and other low fibre things. After a few more days he was able to introduce normal food gradually and it now eating healthily and with more fibre. He also takes a good quality probiotic capsule and multivitamin.
Touch wood he has had no further flare ups and has taken it a a wake up call. He is also eating far more of the food I prepare rather than coming home with a pizza or fried chicken!