Low carbing is pseudo science crap!
A healthy diet incorporates all the nutrient groups in moderation. Carbs don't only provide energy but also protein vitamins and minerals and allow nutrients from other sources to be absorbed too.
It's portion sizes. I'm overweight myself currently losing and my portion sizing estimates are shit I need to weigh stuff.
The only foods I would class as definitely unhealthy are crisps, chocolate and sweets. Even puddings (in moderation and as a treat) contain nutrition.
Were you with your mother at this appointment? Sounds like an OK diet to me providing she varies it and controls portion size. Could do with some more veggies in there but I suspect the gp (given Ltd appointment times) was just giving a rough idea of healthy foods to have as meals as opposed to what your DM is currently eating.
My mum has had to change her diet as it was affecting her health too. It's been interesting discussing with her what she thinks is healthy/an OK portion size to what is ACTUALLY healthy and an OK portion size. Fwiw my dad (a healthy weight) has been telling her for decades that her portion sizes are out of whack and prior to this it lead to food waste as he just couldn't eat what she thought he ought to be portion wise and that was even though he worked a manual job and she a sedentary one! (Old family argument, suspect this has been why my sis and I now struggle with our weight/portions)
Carbs should be the base of the meal equal to veg in proportion, then healthy protein (meat, fish,quorn, beans whatever).
Porridge lowers cholesterol, is low gi (funnily enough in keeping with your Dr friend), and is very filling. Dried fruit (a small handful) would add flavour and provide iron as well as vits and minerals.
A baked potato and beans with a salad (could DM have omitted mention of this so she doesn't 'have' to eat it?) Would provide energy, protein plus vits minerals at a busy time of day.
A meat and 2 veg meal (veg doesn't have to be potatoes therefore only 2 carb portions in the day actually) again perfectly healthy.
Fruit to snack on (again I wonder if veg was mentioned but omitted?)
I'd maybe add a homemade veg soup for an extra bump of nutrition and to fill someone not used to smaller portions.
Also agree sweetener not ideal. But that's another debate.
Absolutely NOT guaranteed to make her gain weight, in fact what's her normal eating pattern?