Unless you're dropping calories, you won't lose any fat. At least carbs give you quick energy and is the body's preferred source of energy.
Going keto for 5 days is a big gamble, in terms of how one's body will adapt. There's also the greater risk of overeating on fats.
At this stage she doesnt care about dropping FAT, she cares about dropping WEIGHT, which even way possible to meet a BMI target, so dehydrating is the way to go also i see starving herself for a couple of days and completely staying off the carbs which hold water, YES she will be severely dehydrated, YES she most likely wont lose any FAT as such, BUT, I see he dropping a couple kg and therefore meeting a BMI target,.
Is it the right way to do things and to go about it, probably not, are there better solutions, long term definately, but for now this is what we have to run with.
SO i would agree dropping all carbs is the way to go, careful monitoring of cals, which is why i have said a couple of shakes per day and 1 maybe 2 meat meals, maybe one red meat with fat the other lean meat such as chicken breast no skin, this should take care of overeating of fats.
Its only 4-5 days so she can do it this way i am thinking, along with some HIIT session, mainly to increase sweating and burning of extra cals compared to LISS which i actually think burns bugger all cals in short term but i agree is good for burning extra cals while maintaining muscle.