I remember I had a similar case a long time ago. So this is how I will explain it to the provider.
So dilated left atrium is referring to a dilated chamber of the heart. There are 4 chambers of the heart, the Right and Left Atrium and the Right and Left Ventricle. If you go to your ICD-10-CM index, go to dilatation, there is no option for atrium, or heart chamber, but, there is for "ventricle" being one of the chambers of the heart. If you go to dilatation>ventricle it takes you to the vague code "cardiomegaly." So code I51.7 is a good option, inform him your book guides you to this code.
Now, if he/she doesn't agree then you can provide code I51.89 Other ill-defined heart disease, that will be as specific you could go.
Hope this helps.