LVEF dropping from 40% to 35% indicates mild decline in heart pumping function after angioplasty. This can happen due to previous heart muscle damage, medication adjustment, or ongoing strain on the heart. It is a matter of concern but not an hard emergency if he is stable and symptom-free. Please consult cardiologist for medicine optimization, strict BP/sugar control, and repeat echo after 3–6 months.

Whether you need to worry or not, will depend entirely on his symptoms and other investigations, such as ECG and Troponin test. Many times, the variation in these numbers could be because of inter observer variability. As many times, The echo might have been done from a different centre at a different times.