Fluid in the lungs is treatable. Of course it needs to be established whats causing it. They may drain it or treat it with Diuretics. But clearly the root cause needs to be established. Its may not be due to the bypass surgery. Its probably serious enough, but should be treatable. Of course his doctor is best placed to advise, as they are in posession of his medical notes and history. They would also have details of any complications he may have. Try keep positive, its a worrying time for you. Keep us posted
Also to add he is diabetic and his blood sugar levels which were absolutely normal has been shooting up during this period