Usually intervention is recommended if the vessel is occluded greater than 70%. Narrow by 30% indicates a 70% opeining, and if intervention is suggested, I believe the blockage is at 70%.
The guidelines by AHA/AAC suggests medication to dilate valves for therapy of symptoms. If there is pain, such as angina (chest pain), not relieved by medication, then a stent implant...if the location, size and so on is not suitable for a stent, then a bypass.
I have a 72% blockage of a coronary artery, and have successfully been treated with medication. There is no cure as your question suggests. The options available just treat the symptoms. Regardless of which therapy, your father should follow a regimen to help prevent any progression of coronary artery disease and that includes proper diet, moderate lifestyle, exercise, etc.
Hope this helps, and if you have any further questions, post a followup. Thank you for sharing.