As you may already know isosorbid dinitrate is an antianginal; it is also a vasodilator. It is used for treating and preventing chest pain (angina). This medication can also be used for lowering blood pressure.
Isosorbid dinitrate is mostly metabolized by the liver. The dosage should be lowered for patient who has cirrhosis. 20mg of isosorbid dinitrate is not a lot. If you are worry, go back to the doctor and ask if s/he is aware of your mother situation. Everyone is different. I cannot tell you whether this is the right medication or dosage for your mother without her current and pass medical history.
If you have any question, you should always go back and ask the doctor. It is better safe than sorry. Answers on the internet may not be totally accurate. Again, everyone is unique. There is no one method that will fit everyone.