So we're on the same page I feel I have to explain what a carb free or Ketogenic diet is and how it works because I think you may misunderstand and/or have misread.
A ketogenic diet is where you reduce the amount of carbohydrates/starch that you consume to such a low level that your body is forced to switch from burning the usual glucose from carbohydrates/starch for fuel to burning fat for fuel (this will automatically happen if carbohydrate/starch free anyway).
If forced to do so your body will use fat as it's main energy source and your liver will convert fat into ketones that your brain will then use for its fuel.
The ketogenic diet is backed by biological science and is used to treat non drug responsive epilepsy in children, it's actually the only thing that is known to treat non drug responsive epilepsy in children.
(It's actually patented as a medical treatment, how you patent a diet is beyond me but that's another matter)
https://en.wikipedia.org/wiki/Ketogenic_diet
To do this you need to drop the carbohydrates/starch consumed to less than 50g per day on the whole, even better is less than 20g or as little as possible, although the level needed to reach ketosis is subjective to each individual person and blood tests are needed to be done unless you have body fat calipers to measure yourself burning body fat without burning muscle as you would be under a normal low carb/calorie deficit diet but still too high to fall into ketosis and PH strips to test your urine ketone levels (although this is just a guideline to blood serum ketone levels).
A low carb/starch diet does not automatically switch your body to burning fat as it's preferred fuel source unless you are Ketogenic, if your carb/starch consumption is low but not low enough to drop into ketosis then all you do is keep burning glucose for fuel but once the consumed carbs/starch is used up as glucose then you will deprive your body of enough fuel to survive from day to day and thus you will become catabolic, catabolic means you will start to break down your own body mass to use as fuel in the form of ATP.
So in a nutshell if (under a normal/conventional diet) your daily carb/starch consumption is lower than your BMR (basic metabolic rate) needs then you will lose weight, that weight loss being taken nearly equally from fat stores and muscle mass (muscle mass slightly more).
Since xmas on a Ketogenic diet I have lost a good stone in bodyfat without losing any muscle mass and that is without any exercise (I'm back in the gym to start building muscle tomorrow/Monday).
So now back to the diet thing and the example of your mother in law.
If your mother in law is on a low carb diet like the Banting diet and somehow she can eat sweet potatoes a plenty then that is not really a low carb/starch diet because as we know sweet potatoes are around 20% carbohydrates/starch (20g per 100g) of which just over 4% is made up of already simple sugars (simple sugars cause an insulin spike and insulin stores carbs as triglycerides/body fat, carbohydrates/starch break down into simple sugars, simple sugars are attributable to heart disease), as I pointed out before regular potatoes have less carbs and simple sugars than sweet potatoes do (have a look at the nutritional make up to see for yourself) so there is something very wrong with eating sweet potatoes and not regular potatoes on a so called "low carb" diet.
And that's without accounting for the unseen/not so obvious carbs in the rest of her diet/meals per day, but because I don't know her diet/meal plan I can't add it all up, all I could do is give examples of everyday foods that contain unsuspected carbs but if the starting point is sweet potatoes then the low carb diet by definition is already out of the window.
What I find very interesting is that your mother in law has been advised by somebody to utilize a low carb/starch diet in respect to her quad bypass and her future cardiovascular health and obvious heart attack potential (quad bypass screams 'genetics', genetics being the #1 factor when talking heart disease, genetics comes way before diet), because advising low carb/starch is defo not the norm among the health care industry the world over, the general consensus (although wrong) is that cardio health is damaged by LDL cholesterol (saturated fat) and not simple sugars from carbs/starch.
The low carb/starch approach is very new (although backed by modern scientific evidence) and can almost be labelled as a 'fringe' concept. (not that diet makes any difference to a quad bypass example, to get to quad bypass levels from diet she'd have to have been eating pure sugar out of the bowl for most of her life, (this analogy is sarcastic but not entirely far from the truth)
The advise of a low carb diet is spot on, but whoever advised low carb/starch yet still lets her eat sweet potatoes clearly doesn't understand nutrition at all.
I can't see the Banting or Atkins diet (pretty much the same thing) being advised by a professional but a Keytogenic diet yes (as keto is backed by scientific evidence where Banting/Atkins is not).
I'd be interested in knowing what kind of meal/diet plan she follows and according to what rational/logic, and also what ratios of macro nutrients she's been advised to consume and how she keeps track of them in order for it to be defined as 'low carb/starch' diet (I have to use a software application to track my consumption and I'm pretty clued up).
Because although a low carb/starch approach is the way to go for ensuring good cardiovascular health, eating sweet potatoes is not (by definition) low carb/starch.
Along the way somebody has fucked up, (whether that be a health care professional or a family member/friend, I have no idea without details) because the fundamental concept is right but the implementation is very wrong.
It's also interesting that somebody is in on the science enough to advise a low carb/starch diet but is ignorant enough of nutrition to allow the consumption of a high carb/starch food like sweet potatoes, this is what I can't wrap my head round. A low carb diet for heart disease and sweet potatoes do not coincide with each other.
Peace.