I totally agree. However, ask them to do FFR on the arteries which is a sensor that records blood flow. At first glance it looks as though the 80% blockage is causing all the issues, but it could be the 20%. If you mention FFR to the cardiologist he will know what you mean. If they give the excuse they don't have it, ask for a medical centre which does.
Stenosis can not be detected by ecg or echo. It can and has shown up now on the angiogram. As the more severe stenosis is located at only one place, the placement of a stent seems to be a good and quick solution for your mom.