Per unit mass, fat has the more than twice the energy of the other two (carbohydrates, protein, and fats). Fats contain 9 cal / g. A dietary calorie = 1 scientific kcalorie. A mango normally weighs about 500 g but let's say the fruit we wish to use is twice that (1000 g). The most energy you can put in your fruit is $ 1000 gram \times \frac{9 calorie}{gram} = 9000 calories $. In this case, it'd simply be a massive lump of lard.
Our bodies preferentially burn carbohydrates for energy and require proteins for cellular construction, so you'll want to dilute your fruit with these other components.
The bar must also contain the micronutrients essential for the healing, probably in proportion to our regular needs for vitamins and minerals but at increased amounts.
All in all, you probably need a fruit composed of all the things we normally need in our diet and approximately in the same proportions as our diet requires. If you weight it a little heavy with fat, then you'd get something like:
- 350 g ~ 3150 calories fat
- 350 g ~ 1400 calories protein
- 300 g ~ 1200 calories carbohydrates
- A few g micronutrients
Modern technological humans discovered a need for concentrated nutrient foods for people with ill health, have problems with digestion, or who are malnourished. You can find this food in your grocery store under the brand name $ Ensure^{TM} $ (among others).
What I see in this 237 g bottle is:
- 6 g ~ 54 calories fat
- 9 g ~ 36 calories protein
- 40 g ~ 160 calories carbohydrates
- 1 g or so of micronutrients
- ~180 g of water
Your fruit would probably have a similar composition.
From personal experience I can also assert that if your fruit is too nutrient dense, it will cause digestive issues as it pulls water out of your body and into the digestive tract through osmotic pressure.