You can probably buy potassium supplements from your local pharmacy but you need to be aware that unlike food sources of potassium supplements can lead to toxicity and hyperkalemia even in healthy adults. For this reason I'd suggest she overcomes her fear of carbs, at least for the time being anyway, and eats her veggies. And although she may protest her best option, particularly if she's worried about gaining weight, is probably the much maligned white potato because they're a good source of potassium and with just 69 kcal per 100g, or 690 kcal per kilogram (2.2 lbs), there's no way on Earth she could eat enough of them to get fat.
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements (2006): Potassium (p. 371.)
Dietary Reference Intakes: The Essential Guide to Nutrient Requirements (2006): Special Considerations (p. 376).
Nutrient data for 11354, Potatoes, white, flesh and skin, raw.