Straight-up no frills paleo ought to do it ...
Meat, fish, proper fat, eggs and vegetables.
No grains, beans, pulses, nightshades or dairy. Bring one at a time back in gently and observe the effect. Come back to a "new normal" with an understanding of what works and what doesn't. So many folks are dairy intolerant, yet find yogurt and A2 type dairy is fine; gluten intolerance is more widespread that you'd think, but not all forms are too bad and there are forms of bread which are properly made and perfectly good to eat.
Best of luck to the Mrs. It really is about the gut and good gut health. Everything else "just works" once the gut is back! Feed the gut, don't fibre it to death!
... just meat and vegetables does sound good, but if she's going that far (to reiterate my first point), drop out bread, beans and dairy, too. Fermented dairy ain't too bad so yogurt is okay.
DO NOT skimp on the fat. Inevitably, these diets lead to lower carb intake and so the low-carb "high" but you soon find lack of energy, so fat intake is crucial! Fat, not sugar. She can read about metabolic flexibility online, but suffice to say, humans can process both sugar and fat for energy, but we're better if we switch between the two. Fat, it should be proper fat - from animals, coconut oil as a second best, but very good second best.
For illustration, for fencing competitions, I'll have an avocado before driving to the competition. I will drink water through the morning (because competitive sports dig do make you sweat), not energy drinks. Lunchtime, I'll eat Jerk Chicken and have a couple of bananas. In the afternoon, I'll drink water with some salt in. I can go all day on fat and just fat. I don't suffer sugar highs and associated crashes. Those bananas really help with muscle fatigue and potential cramps; yes, they're carby, but my body won't be using that energy 'til way after the competition. Once home, it'll be steak & mushrooms with a cream sauce for dinner.
Point of illustration I: humans are fat burners. Protein is the building block, fat the energy source.
Point of illustration II: we don't actually need a lot of intake, even through periods of high, frenetic and prolonged activity.