Freezing actually preserves the nutrients!
If you cut up carrots and raw beets, store them in some water so they do not lose their crunch and hydration. A little bit in the bottom of the container is perfect...some people soak them in the cold water which works too.
If you mom is taking any blood pressure medication, please be sure there are no food/drug interactions and if so avoid those foods. Focus on plenty of potassium-rich produce and avoid salty foods (most processed, frozen foods and take-out or restaurant foods are loaded with more salt than you need in a day).