When the EF is below 29% very often there is a failure of the heart to pump into circulation the amount of blood received from the lungs. That may be intermittent or sustained for a longer period of time, and as result blood backs up into the lungs and fluids leak into the lung tissues causing pulmonary edema (fluid in the lungs).
Did your father have an enlarged left ventricle at the time of his CABG? Does you father have an enlarged LV currently. Usually when the LV is enlarged that will cause weaker than normal contractions and as a result the EF will be lower than normal. Several years ago I had pulmonary edema and the symptoms were shortenss of breath, dry cough, hyperventalating and diagnoses was mild edema...so your father may not be having episodes that is medically significant other than there is a serious underlying cause.
Your father's heart may take more time to return to normal functionality with an EF of 50 to 70%...that is the amount of blood pumped with each heartbeat. The EF can be increased medically and/or reducing the heart's workload giving it relief and more time to heal. There are other causes for lung edema, but the low EF strongly indicates it is the left ventricle's ability to strongly contract to be the problem.
Hope this helps, and if you have any further questions or comments you are welcome to respond. Thanks for the question, and I wish your father well going forward. Take care,
Ken