Underweight dog with heart murmur

Question
My friends have a dog that I am getting concerned about. She's part Bichon Frise part Shih Tzu. She has a heart murmur and is underweight. They also have a toy poodle and they free feed, so they're both on the same food and they eat when and if they want. I am looking for a way to help her put on weight in a way that is also safe for her heart.

Answer
Are you sure she is under weight, and not just thinner than the commonly overweight dogs?  Evaluate the dog as illustrated in this link, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx  You may want the vet to confirm your judgment. Adjust the dogs food and exercise as needed to reach its ideal body condition.

If she is underweight and refusing to eat enough to gain weight, she may have other problems the vet needs to check for.