Are you saying that she has chest pain and an irregular heart beat. As Jerry says can you give us a little more info...like how tall is she, how much does she weigh, does she smoke, does she drink, is she on any medications, if the doctor is telling her to eat healthier how is she eating now, does she exercise, what does she do each day? If we can get answers to these little questions i bet someone can give you some good answers...
We don't have enough information to have much to relate to. Keep in mind, we are not doctors, we are all patients who draw from our personal experience to try to find helpful information.
You didn't mention blood pressure. High BP can cause heart enlargement and should/must be treated. The treatment is rather simple and the medication in generic forms are inexpensive.
Of course, a health life-style, including diet an exercise if critical to us all for good heart health. Easy to say, hard to do, but I encourage you to talk with you mother about these items if you think they may help.
Age 45 is young for heart problems driven by aging, still heart problems can develop at any age, including in the womb (unborn babies).
I assume given she had an echo test that any blood pressure issue has been discussed, but that's all that I can offer at this point.
Give her your support, encouragement and love... they all help to make everyone's heart healthy.