Normal EF is 50 to 70%, and that indicates the percentage of blood pumped with each heartbeat. The risk of a low EF can lead to heart failure. Heart failure happens when the supply of blood received from the lungs is not effectively pumped into circulation due to weak contractions of the pumping chamber. Uncorrected the heart will enlarge and that weakens the heart further and causes a risk of cardiac arrest.
A silent heart (no symptoms) is not unusual for an individual that is diabetic. Apparently, there was some heart wall impairment (hypokinesis) that reduced the heart's ability to pump effectively. About 6 years ago, I had a silent heart attack that caused heart failure because there was hypokinesis (heart wall movement impairment) that reduced the contractility of the left ventricle to an EF below 29%. The underlying cause was ischemia (lack of blood flow) due to occluded coronary vessels.
Your doctor would be the best source to decide if further testing is indicated and what medical treatment is appropriate. My medicine was/is an ACE inhibitor that dilated the blood vessels, beta blocker to maintain slow, regular heartbeats and aspirin. Currently, my heart is functioning properly with a normal EF (59%). I don't/didn't have diabetes and that certainly is a consideration for treatment and prognosis for your father's condition.
It is possible your father's heart problems can be effectively treated with medication...your doctor has your father's health history, etc. and should be able to make the proper health care decisions.
Thanks for sharing, and if you have any further questions you are welcome to post. Take care.