To answer your question directly the answer is NO.
Now let me detail why and how you can lose fat and maintain muscle in 4 points:
I Rate of body fat loss
1 gram of fat is approximately 9 calories.
To consume one kilogram (2.2 pounds) of fat you need to create a deficit of 9000 calories.
Meaning that if you plan on losing one kilogram of fat in a week you'll have to eat a total of 9000 calories less than what you spend which is roughly 1300 calories a day.
If you add cardio/weight training (which is tremendously more effective) to the point where your calorie expenditure is 3000 calories a day, you'll need to eat 1700 calories a day for a week... THEORETICALLY.
This is not taking into account the variations in metabolic rate that occur in a deficit for example (eating less makes your body consume less in summary). Not even taking into account the loss of muscle to get energy.
II So how did my friend lose so much ?
Water my friend.
Water mass is very variable and can change really fast overnight.
I myself can lose 1 kilogram in one day without being on a deficit.
It happens if I go out a little, drink some alcohol (even two beers are enough). I am therefore a bit dehydrated and lose water really fast.
I've lost up to two kilograms in one night (even with less than 10% body fat).
Your friend is probably losing glycogen and water fast. That's it.
III What can I do to trigger faster results ?
Doing what your friend does is not necessary. There are a few ways to lose mass fast which I wouldn't recommend:
By eating mostly fats and protein you deplete your glycogen levels. To be stored inside the muscle, it needs water. On that kind of diet you lose glycogen and water very fast.
However as soon as you start eating carbs again your glycogen levels rise up as well as muscle water retention.
Your weight will get back to normal really fast.
- What your friend does, which is basically losing a lot of water but then getting it back when diet is back to normal.
This isn't sustainable and your weight will go back to normal fast.
IV Then what can I do to trigger sustainable results ?
a. Slow and steady fat loss.
As boring as that sounds, it seems to be the best way to keep a good amount of muscle mass and lose fat.
Weigh yourself daily upon waking up. Aim for 0.5-1% bodyweight loss a week (if you're not obese - in that case it will be higher).
If your weight loss is higher, eat more. Otherwise you're losing more muscle than you think.
If you don't lose weight, check your weightlifting program, macros/calories, sleeping habits and stress levels.
b. Protein intake
According to this study, having a higher protein intake for the same total calorie intake will make you keep more Fat free mass. Try consuming 2g protein/kg of bodyweight.
Exercising properly will make you retain more muscle mass too according to this paper. So pick a good weightlifting program that you enjoy and stick at it. One small very personal advice on this: the simpler it is, the more efficient it will be. Try to find one with very few full body movement (Starting strenght... ).
d. Lower sodium intake.
Water retention will add mass around the waist. At very low body fat percentages, I personally noticed that after a high-sodium eating meal, I woke up with less definition in my abs. Considering of course that caloric intake wasn't higher.
Therefore try getting less sodium. Progressively from what you get now.
Caution: According to this paper, adverse effects can happen on very low sodium levels
e. Sweat it out
Lose water. And drink a lot to replace what you lost in order to avoid water retention.
You can find online quite a bit of research on sleep deprivation and testosterone levels. This hormone is highly correlated to muscle repair and fat mass. This article talks about the effect of sleep reduction on testosterone.
Sleep well to get the most out of your training and diet.
To sum it up:
- There is no way to lose weight fast (except for obese people) without having adverse effects on health or muscle mass.
- Get in a slight calorie deficit to lose around 0.5-1% bodyweight a week. Weigh yourself daily. If progress stops, check calorie intake, training, sleep.
- Drink water and reduce (safely) your sodium consumption
- Sleep well
- Lift weights
- Repeat until goal is achieved
One quick thing on abdominal tightness:
I never do abs and am lucky enough to have a low bodyfat percentage. Abs are very small muscles. Training them will not make you consume a lot of calories. Train bigger muscles and do some abs/vacuums when you get to less than 10% bodyfat.
Good luck with your goals ! I hope this helped !