I'm actually an excellent example of this - due to digestive and endocrine imbalances I've had quite a range of symptoms and conditions in my life. Each on their own might benefit from following distinct diets but combined these conflict in certain areas so they require a bit of a balancing act.
Something I suffered with from quite a young age was impaired absorption of nutrients, in particular Vitamin C and iron levels. Whilst alot of people don't need to eat meat to get sufficient iron, and are fine with plant sources and maybe a good supplement, I find that if I totally avoid meat I tend to get a bit low in iron and my energy levels can suddenly drop. Vitamin C is also really important for proper iron absorption so I have higher requirement than average for both nutrients.
Unfortunately, due to an overly sensitive digestive system, I have problems digesting fructose so can't eat too much raw fruit. I therefore have to generally stick to lower sugar fruit such as berries and orchard fruit, and I find that cooking them helps me digest them. In addition I eat plenty of fresh vegetables and treat these as my main source of vitamin C.
I have also suffered in the past from skin breakouts due to disregulated hormones, but find that if I follow a meat and dairy-free diet my hormones are much more balanced and my skin is perfect. However I have to balance this with my need for iron, so do eat meat but only once or twice a week.
Getting enough protein on a vegan regime is tough, the main sources are pulses, tofu, soya protein, tempeh, seitan (which I can't have due to it being wheat derived) and nuts and seeds. Unfortunately my sensitive digestive system can't handle too much soya milk so I tend to rely on pulses and occasional tofu for main meals and nuts and seeds in snacks. I have a protein shake in the morning with breakfast, but of course being dairy intolerant I can't have whey protein! So I have rice protein instead. I also have oily fish a couple of times a week for some extra protein and omega three fats.
Energy and weight wise I feel best on low GI carbohydrates balanced with healthy proteins but unfortunately my intolerance to gluten, and some other grains, means I can't eat the nice super low GI wholegrains such as oats, rye, buckwheat and spelt. Instead the only wholegrain my body can cope with on a regular basis is brown rice, which does get a bit dull, so I do allow myself some of the processed higher GI gluten-free pastas and breads.
This approach may sound overly complex and quite frankly pretty restrictive, but it isn't a diet I suddenly switched too overnight. It has evolved over time and I fine tune it according to how I feel. Plus I only follow these restrictions eighty per cent of the time, allowing myself offplan indulgences, but infrequently enough as to not de-rail my health.
The point is, that we should all base what we eat around some healthy fundamental principals, but we then need to pay attention to how our body responds to food, both positively and negatively, and adapt accordingly. Just because your colleague feels great on a low-carb diet doesn't mean it will suit you, but that shouldn't stop you trying it out ... just remember finding that right diet can be a balancing act, and if you don't feel good you probably need to change something.