We've had several cavaliers with MVD over decades of having the breed, and all of those with MVD eventually entered congestive heart failure (CHF). Prior to CHF, he give our MVD dogs supplements to keep the heart healthy. There is a list of them here:
http://cavalierhealth.org/diets.htm#Heart-Healthy_Supplements:
-- Vitamin C and Vitamin E and CoQ10 and Udo's Choice Oil 3.6.9 Blend and fish oils.
-- Sogeval Laboratories' Antiox-Ultra 5000.
-- Thorne's Bio-Cardio or Vetri-Science's Vetri-Cardio Canine Chews.
-- Standard Process' Canine Cardiac Support or Cardio-Plus or Cardiotrophin PMG or Cataplex E or Vasculin.
-- Health Concern's Flavonex.
-- Corvalen Ribose or Pure Encapsulations Ribose.
But once the dog enters CHF and is prescribed medicines by the veterinarian, we stop giving any supplements which would conflict with or compound the effects of the prescriptions. So, once the dog is being given a diuretic and/or an ACE-inhibitor (Fortekor) and/or spironolactone (Prilactone), the supplements we still would be giving the CHF dog are: the vitamins and the oils and the anti-oxidant and either the Bio-Cardio tablets or the Vetri-Cardio chews.
Obesity is very bad for CHF dogs. So, they should be kept at a moderate weight -- but not thin. There are a couple of body condition scoring charts here:
http://cavalierhealth.org/diets.htm#Body_Condition_Scoring All cavaliers, but particularly those in CHF, should be in the mid-range for either of these charts.
They need daily exercise, but not too strenuous or too long. A couple of daily walks would be good, but don't force the dog to walk to far or at all if he indicates she doesn't feel well enough. At some point, the dog may not be able to walk outside, but she still needs the stimulation of fresh air and smells. So, consider a stroller for her to travel in on "walks" each day.
The CHF dog may want to lie on a hard surface from time to time, so one should be available for him. Kate H. makes an important point of raising the food and water bowls a little bit, to make it easier for the dog to reach their contents.
You did not mention that she is being given Vetmedin (pimobendan). At some point, the vet probably will recommend adding it to her daily cocktail of pills. For many CHF dogs, Vetmedin can really improve the symptoms and give the very sick dog new vigor. Vetmedin should not be given until after the dog is in CHF, and even then, it probably is better to wait a bit to see if the other medications improve the symptoms before adding the Vetmedin.